Olfactory Chemistry: Norlimbanol- Mirror, Mirror Who’s The Woodiest Of Them All?


Today’s perfume cliché was last year’s game changing raw material. One raw material, Norlimbanol, has followed this trajectory from its early exciting use in 2001 to today’s overused woody basenote. From a chemical point of view there are a couple of interesting aspects about its structure.

norlmibanol enaantiomers

One is that the structure you see above (+)-Norlimbanol is one of the most powerful woody notes in all of perfumery. If you just change the geometry by taking the bonds which have the solid wedge or the dashed wedge and reverse all three of them so the solid wedges are now dashed and the dashed wedge is now solid. You have (-)-Norlimbanol. So it is the same structure but the solid wedges are coming out of the page and the dashed wedges are going behind the page. These two structures are what are called enantiomers. They are mirror images of each other. That enantiomers have dramatically different properties is not just confined to perfume. This phenomenon extends to drug discovery as well. There are drugs where one enantiomer has the positive effect and the other enantiomer causes a bad side effect. Separation chemistry has evolved so much that separating these isomers has become easy and it allows for a chemist to isolate the specific enantiomer with the desired character almost at will. This is why even though it is a single enantiomer it is one of the more economical ingredients to be found in perfumery.

Norlimbanol was discovered by Firmenich chemists and patented in 2000. In 2001 perfumer Olivier Cresp would use it in two of the best designer perfumes ever released. M. Cresp didn’t just use Norlimbanol by itself he mixed a potent mix of Norlimbanol and two other synthetics Ambrox and Z-11. The two perfumes this mix was used in were Dolce & Gabbana Light Blue and Paco Rabanne Black XS. If the idea of strong long-lasting woody bases has become a cliché these were the two perfumes which popularized the concept. Nowadays Norlimbanol is often used by itself and because it has a great longevity and projection perfumers add it in because they believe this is what consumers want. I would say that the perfumer who takes the time to balance it out with other raw materials to deliver a specific effect can still use Norlimbanol effectively and creatively. Unfortunately those perfumes are few and far between especially on the department store counter. Next time you are there take a sniff of Light Blue or Black XS and keep in mind how Norlimbanol is used there. Then pick up any other perfume next to it that has a woody base and you will instantly see the difference.

Norlimbanol may have become trite due to overuse but it is still one of the most versatile and interesting synthetics on the perfumer’s palette.

Mark Behnke  

Olfactory Chemistry-Damascones

Part of the purpose of this series is to illustrate how the most simple of changes can have dramatic effects on not only the odor profile but even the stability of the molecules themselves. The class of molecules known as Damascones portray both of these qualities.

ionone damascone

As you can see the Damascones are closely related with the only structural difference being the reversal in position of the C=O and double bond. As I wrote in the installment on Ionones those molecules all carry a variation on iris and woods. The Damascones are one of the key natural products which make up rose oil. The name itself comes from the Damask rose from which it was isolated and the structure determined. These molecules impart a dark rose or fruity quality when used.

beta ionone damascone

The other difference is the stability of the molecules. Ionones are a work horse molecule in perfumery and are used far and wide. The Damascones were used and, because of their nature, a little goes a long way but they are much more prone to degradation. In a paper presented at the 2000 meeting of The International Federation of Essential Oils and Aroma Trades (IFEAT) Dr. Robert Bedoukian showed this difference. Beta-ionone and beta-Demascone were left open to the air in a clear glass flask for 24 days taking samples at day 6, day 17, and day 24. Dr. Bedoukian was looking for a common oxidation product which forms called a peroxide and measured the levels of peroxide. Beta ionone showed values of 20, 60 and 85 mmol/I of peroxide on days 6,17, and 24. Beta-Damascone showed levels of 45, 130, and 150 mmol/l over the same time points. You can see that the rate of decomposition is more than twice as fast for the damascone than the ionone.

Even with all of that Damascones are the key ingredient in three of the most important fragrances ever made. One of its earliest uses was in Guerlain Nahema in 1979 it gave texture and depth to the central rose in Nahema that makes it to my mind the best rose perfume ever. Damascones also played a part in Christian Dior Poison in 1985 and in the perfume considered the “best ever”, 1981’s Shiseido Nombre Noir. None of these have survived reformulation unscathed as IFRA called out the Damascones as sensitizers and the possibility of being reactive. The levels of Damascone able to be used was reduced to less than 0.02%. Which especially for Poison and Nombre Noir, which used the damascones in overdose, forced a significant effort to reformulate in the case of Poison and Nombre Noir was simply discontinued.

Mark Behnke

Title Image: Shattered Rose by 8manderz8

Olfactory Chemistry: The Physics of Supercritical Fluid Extraction- It’s a Gas, Gas, Gas

For years the extraction of the essential oils from natural sources was done via extraction in hot ethanol and distillation of the resulting solution to collect the essential oil. This process due to the heat used for the extraction and the distillation causes loss of some of the ingredients which are heat sensitive or reactive with alcohol when it is heated to boiling this is called denaturing. What this means is the process does not get the whole spectrum of ingredients that come from the natural source. The only alternative to this steam extraction and distillation process was the very labor intensive enfleurage which would capture a fuller amount of the natural products but still not everything.

Then along came supercritical fluid extraction. This is a process where a solvent which exists at a gas at normal pressure and ambient temperature when placed under pressure and cooled, liquefies. If there is a source of essential oils covered by the cold liquid it extracts everything out of it. After the extraction is done the liquid is transferred into a vial where it is allowed to return to room temperature and pressure. This turns it back into a gas leaving behind the essential oil. Are your eyes glazing over yet? This is always where I see the light going out of the eyes anyone I have ever tried to explain this to. You know the whole adage about a picture being worth a thousand words? Well the video below which comes from Mane describing their Jungle Essence procedure is worth a million words as it is a beautiful way of showing how this is done.

The solvent used in the video is probably primarily butane, the gas used in cigarette lighters. When used as the solvent in supercritical fluid extraction it performs well. The first solvent used was carbon dioxide and those early essential oils were labeled CO2 to indicate that they were extracted with that. Now all of the aromachemical houses have worked to perfect different blends using other blends mostly using a high percentage of butane.

When I was at the Mane presentation at Pitti they showed the video above and they passed around examples of raw materials which were extracted the traditional steam distillation way, using carbon dioxide as a supercritical fluid, and using their Jungle Essence blend. It was striking to see how much more there was in the supercritical fluid extractions. The most striking was an extract of hot Szechuan pepper. A glass of the ground pepper pods was passed around followed by a mouillette of the Szechuan pepper Jungle Essence. If I was blindfolded I wouldn’t have been able to tell the difference. Petitgrain was also an interesting example because in that case the essential oil realized by steam distillation was only very slightly different than the Jungle Essence version. That showed me that supercritical fluid extraction is not always the best choice.

I hope that the video above especially helps you to understand the technique much better than my overly technical second paragraph and the next time you see a note which says something like sandalwood EO CO2 you’ll understand why it might smell a little different.

My thanks to Mane for the video I really do think it is very well done.

Mark Behnke

Olfactory Chemistry: Ambrox-Chemical Gold


One of the biggest perfume synthetic raw materials is Ambrox. I mean big in both senses of the word. Ambrox is widely used as one of the most common basenotes. Ambrox is also a big molecule as you can see below. It is the size which allows it to linger on the skin for hours and hours. Like many of the synthetic molecules in this series Ambrox came about as a synthetic equivalent to an expensive natural ingredient.


The natural ingredient I am talking about is ambergris. Ambergris has become so prized that it has been called “floating gold”. The price of ambergris has done nothing but go up so synthetic chemistry comes to the rescue again. Natural ambergris was analyzed and Ambrox was determined to be one of the major components. The synthesis was accomplished via a process called semi-synthesis. It is where you take an easily isolated natural product as an advanced starting material and then over a few steps you transform it into the desired product.


For Ambrox the starting material is an isolate from clary sage called Sclareol. It takes six steps to convert Sclareol to Ambrox and it is still pretty labor intensive. As a result ambrox is not one of the cheaper synthetics you can employ, although it is significantly more economical than ambergris. There is a more efficient synthesis starting from a natural product isolated in labdanum but even though it is two steps shorter the chemical reagents necessary to carry out the transformation are more expensive and that makes the shorter more chemically efficient synthesis less economically efficient.

Ambrox made its first widespread impact as a component of the base of Dolce & Gabbana Light Blue. Ambrox would go onto star in Geza Schoen’s Escentric Molecule 02 just as a single dilution of pure Ambrox. If you try that perfume you will see that Ambrox carries some of the briny musky quality of ambergris but it also has significant characteristics of light woodiness. It is that latter quality that really has been used by perfumers to really bring Ambrox to the foreground. Two of my favorite uses come from two different Editions de Parfum Frederic Malle perfumes by perfumer Dominique Ropion; Geranium pour Monsieur and Portrait of a Lady. In Geranium pour Monsieur it provides the austere muskiness at the base. In Portrait of a Lady the woody aspect is more pronounced. Just those two perfumes by one master perfumer show how much versatility this aromachemical has. I also think there is a downside to Ambrox when it is used poorly it often has an overbearing unbalancing effect on a perfume. There have been perfumes I have liked until the Ambrox explodes upon the scene obliterating any nuance. It makes me wary when I see it on the ingredient list because if it is not used well it can single-handedly ruin a perfume for me.

The bottom line is Ambrox is here to stay and over time perfumers have learned how to use it in multiple creative ways. When it is used well it can be chemical gold in a perfume.

Mark Behnke   

Olfactory Chemistry: Lactones- Peaches and Cream

“I’ll take cream in that” is a common phrase when talking about the way you like your coffee or tea. When a perfumer wants to add creaminess to a perfume they are composing they mostly turn to one class of compounds called lactones. The names of these compounds are derived from the Latin phrase for milk, lac lactis.


Lactones were discovered in the early 1900’s as chemists found a way to cyclize esters. A beneficial side effect of this cyclization was it took esters that couldn’t be used for perfumery because they were so short lasting because of their volatility. The cyclic form could last on skin for hours, even days in some cases. The very simple case is shown above as methyl propionate is the ester on the left and when it is cyclized it is called gamma-Butyrolactone. Lactones are also found naturally, most prominently in tuberose. There are so many lactones in tuberose new ones have been discovered as recently as 2004.

peach coconut lactone

Almost from the moment they were discovered the lactones became key components of perfumes. One of the most influential, in perfumery, is one called Peach Lactone. Chemically you can see it is an analog of the gamma-lactone with a long carbon chain attached to it and is called gamma-Undecalactone. For no reason I have ever been able to understand it is also called Aldehyde C-14 because it is not an aldehyde, it doesn’t even decompose to an aldehyde. The nomenclature craziness continues as a smaller lactone, gamma-Nonalactone, is called Aldehyde C-18 and also called Coconut Lactone. This kind of confusing way of referring to the molecules drives the chemist crazy. Don’t even get me started on Aldehyde C-16 which is neither lactone or aldehyde nor structurally similar to the two above.

Peach Lactone is the key ingredient in one of the best perfumes of all time, Jacques Fath Iris Gris. Dawn Spencer Hurwitz used it as a key component of her reconstruction of this fragrance for her recent Scent of Hope. Peach Lactone forms a gauzy fruity layer carrying smooth creamy components. Peach Lactone is also found as the source of that fruit in Guerlain Mitsouko. In almost every great case I can think of if you smell peach in a perfume this is probably the chemical behind it.

massoia lactone

One of the most amazing uses of lactones has come recently in 2011’s Hermes Hermessence Santal Massoia by perfumer Jean-Claude Ellena. M. Ellena takes Massoia Lactone and uses it to bridge the natural creamy qualities of fig and sandalwood. Massoia Lactone besides the creaminess also has a rich caramel aspect and it is this which creates a dulce de leche accord in the middle of Santal Massoia.

If you smell fruit and cream in your perfume it is lactones which are probably responsible.

Mark Behnke

Olfactory Chemistry: Alicyclic Musks- Breaking the Cycle


The most current iteration of musk molecules form the class known as alicyclic musks. In all of the previous versions of musks they were variants of the original nitro musks or they were synthetic counterparts to the natural molecule muscone. In the alicyclic musks these finally comprised a new class of molecule uninformed by previous musk molecules.

rosa and cyclomusk

The first alicyclic musk would be discovered in 1969 by scientists at International Flavors and Fragrances (IFF) and it was called Rosamusk. Even at a fragrance house like IFF, Rosamusk found no enthusiasm for its use despite its interesting rose and fruit character over musk. Rosamusk was so overlooked that older texts on the musk molecules will tell you that it was BASF’s molecule Cyclomusk, discovered in 1975, that was the first alicyclic musk. Cyclomusk gets the press because it is a muskier smelling molecule but it is the fruity floral aspects of Rosamusk which have become the defining characteristic of this class of molecules.

helvetolide romandolide

It wouldn’t be until the 1990’s that Firmenich would make the two molecules which have come to represent this class in perfumery; Helvetolide and Romandolide. If you look at the two molecules above you will notice how similar they are and how they were influenced by the molecules which came before. Helvetolide takes the two CH3, or methyl, groups from Cyclomusk and forms a hybrid with the basic structure of Rosamusk. Romandolide takes the structure of Rosamusk and adds on the same molecules at the end that are present in Helvetolide. These are good examples of what all synthetic organic chemists do to produce a desired effect. We look at what has come in the past and if the chemistry allows we will put these fragments together to see if we can make something more useful. What worked in the case of aromamolecules also works in drug discovery.

helvetolide analogs

Ref: Chemistry & Biodiversity, Vol. 1, pg. 1975 (2004)

As you see above the structures of these alicyclic musks differ in the nature of the group on the six-membered ring. It can be quite remarkable how just moving the groups around can have a significant effect on the odor profile. In the figure above there is a collection of derivatives of Helvetolide and Romandolide and you can see by the descriptions just how much removing or shifting a methyl group around the ring can have.

As I said these molecules have a stronger fruity and floral character than other of the synthetic musks. One of my favorite descriptions of Romandolide comes from perfumer Frank Volkl who in a 2012 article in Perfumer & Flavorist says, “It’s the marmalade within a fragrance. For me it’s the type of musk that adds a little bit of fun to the fragrance.” I think that is really the most striking aspect of the alicyclic musks as these are the “fun” musks. They still carry that identifiably musky quality but the fruity and/or floral facets make them lighter in both heft and intention. Perfumer Alberto Morillas uses a high concentration of Helvetolide in Lancome Miracle and Romandolide features in Rochas Absolu by perfumer Jacques Cavallier.

The whole story of the musk molecules is perhaps the best chemical story in all of perfumery as it illustrates the developments of synthetic aromamolecules for the last 100 years.

Mark Behnke

Olfactory Chemistry: Macrocyclic Musks- A Whiter Shade of Musk

When it comes to musk it is not like we as chemists don’t know the chemical structure of the natural source. In 1926 Leopold Ruzicka isolated and chemically identified the molecule found in the musk deer which primarily gave musk its smell and he called it Muscone.

muscone exaltone

In the structure above what you see is a 15-membered ring of carbon and importantly the bold wedge attached to the CH3 represents a methyl group which is coming up from the plane of the page. If that wedge was dashed that would indicate the geometry was going behind the plane of the page this is how we show three-dimensional geometry on the page. This orientation is very important because if that wedge was dashed and the methyl group was oriented differently the odor profile of muscone is significantly lowered. It is exactly the difficulty of getting this geometry just right that gives synthetic chemists so much difficulty in making molecules easily. In 1926 Dr. Ruzicka didn’t even try because he discovered if you just leave the methyl group off and  synthesized the rest of the large ring you still had an acceptable musky profile. Thus was the first synthetic macrocyclic musk created, Exaltone.

musk graph

These all-carbon large rings were difficult to synthesize but if you replaced one of the carbons in the ring with oxygen or added additional oxygen, those were much easier to make. These molecules are called lactones and the smaller versions of these also play a large part in perfumery.  As you can see in the table above the same issue with the muscone still existed as if the methyl group was pointed away the odor profile was much more muted.

nobel and grubbs

Dr. Robert Grubbs and Nobel Prize

The big breakthrough for the all-carbon macrocyclic musks came in the 1990’s when Prof. Robert Grubbs published a technique called catalytic ring-closing metathesis. That’s a lot of words but what it comes down to is now there was a chemical reaction which allowed a synthetic chemist the ability to form almost any size carbon ring imaginable. This is one of the most powerful synthetic methodologies of the last thirty years and Dr. Grubbs received the 2005 Nobel Prize in Chemistry for this work. It fired the imagination of chemists everywhere and in the fragrance industry it sparked a bit of a race into which firm could produce and patent the best synthetic musk.


Alberto Morillas

As for the use of these macrocyclic musks these are the musks most commonly referred to as “white musks”. The term was coined by perfumer Alberto Morillas as he combined a number of these musks to create a “cotton and linen” accord for Emporio Armani White for Her in 2001 and called it a white musk accord in the press release. M. Morillas would use the same white musk cocktail in one of my all-time favorite fragrances Thierry Mugler Cologne later that year.

These musks are also the musks that some people are not able to discern in a fragrance. We term that as being anosmic to musk but that is far too general a term. These very large molecular weight molecules hold a special property that we don’t understand very well which causes certain people to not detect them. The same people who can’t detect these macrocyclic musks can often smell either of the polycyclic or nitro musks and anosmia to those is far less common.

Mark Behnke

Olfactory Chemistry: Polycyclic Musks-What Clean Smells Like

One of the best things about science is it is always evolving and chemistry is no different. As a synthetic chemist I am always looking for the next new reaction that will allow me to easily make the next new molecules I am interested in. What is true for me as a medicinal chemist was also true for the chemists who worked in the fragrance industry. In the post-World War 2 economy there were a lot of chemical by-products being formed and clever chemists were using them to develop new plastics and pharmaceuticals and, yes, aromachemicals. Along with new chemical techniques allowing a chemist to make another ring of atoms fused to the same ring used in the nitro musks the polycyclic musks were born.

galax phant

In 1951, the first polycyclic musk was synthesized by Kurt Fuchs and it was called Phantolide. It didn’t have a very strong odor but it had incredible stability and ability to stay concentrated even in water it became a natural to be added to detergents as this would stick to the clothes after washing. This was the main use of polycyclic musks for many years until 1965 and the synthesis of Galaxolide by M.G.J. Beets at International Flavors and Fragrances. As you can see above Dr. Beets used the new synthetic methods to take the two groups on the right and cyclize them. His hypothesis was if he kept the oxygen in a similar spacing as it was in in Phantolide he might make an improvement, and he did. Galaxolide retained the stability and properties that made it a good detergent additive but it now also had a more concentrated odor profile and could also be used in perfumery. Perfumer Sophia Grojsman would end up using it in a 21% concentration in her masterpiece Lancome Tresor in 1990.

fix cash

This would open the door for other chemists to find other polycyclic musks and when you make the simple change of making the five-membered ring on the left of Phantolide a six-membered ring you get Fixolide. When you completely change the ring on the right-hand side of Phantolide you get Cashmeran. If you want to smell what these three molecules smell like together The Body Shop’s White Musk contains all of these. If you do that you will understand why these are referred to as the “clean” musks as they evolved from their beginnings as laundry detergent odorants to key components of the “clean and fresh” movement in perfume.

Mark Behnke

Olfactory Chemistry: Nitro Musks- From Boom to Musk


There is no ingredient more ubiquitous, and important, in perfumery than musk. The uses of all of the chemical derivatives of musk have resulted in the expansion of the perfumer’s palette dramatically. From a chemist’s point of view the story of the evolution of the synthetic musks is a still developing chemical tale which has now spanned three centuries of chemistry and the inherent advances in new ways to put together molecules. It almost seems that with every new advance in chemistry it wasn’t too long before someone found a way to make a musk aromachemical with that new methodology.


Musk Deer

The reason for needing synthetic musks is because the natural source is a living animal, the musk deer. For many years these animals would be hunted and the glands used to secrete the natural musk would be removed from the animal, killing it. The collected tissue would be dried, then opened to harvest round fatty nodules which would be tinctured. To get one kilogram of these nodules it could take as many as 50 musk deer to be killed. Musk grains were worth twice their weight in gold and because of this value the hunting of the musk deer drove them to the brink of extinction until they were placed on the international protected species list in 1979. There is still a legal quantity permitted to be harvested but it is of such a small amount that perfume use is not high on the list for it.

musk 1

Even though harvest of natural musk wasn’t outright banned until 1979 the cost of it would cause early aromachemical experimenters to look for more cost-effective replacements. In 1888 the first synthetic musk was discovered but Albert Bauer wasn’t looking to make an aromachemical he was looking to make an explosive. There is perhaps no single molecule which has led to more rapid advances in chemical synthesis than tri-nitro toluene or as it is more familiarly known, TNT. From the moment of its discovery in 1863 organic chemists were looking to make changes to make a better molecule to go boom. Hr. Bauer was no different and as you can see in the diagram above he made one change to TNT and found something that did not explode, except in your nose. I am always amused when I read the 19th century chemical literature because contained within the experimental descriptions is how a molecule smells, and often, how it tastes. There was no OSHA back in 1888 to protect Hr. Bauer from himself. Because Hr. Bauer followed his nose he realized he might have found a different lucrative market instead of ammunition. By improving the synthesis he could produce what would come to be called Musk Bauer for about $500 per kilogram.

musk 2

Hr. Bauer would spend the next ten years perfecting three other musks; musk xylene, musk ketone, and musk ambrette. This class of molecules would come to be known as nitro musks. Each of these had different scent profiles and would be used as the predominant source of musk in perfume for almost 100 years. In 1921, Ernest Beaux used a cocktail of nitro musks, but primarily musk ketone, in Chanel No. 5. Musk ambrette is the key note in Francis Fabron’s original formulation of Nina Ricci L’Air du Temps, in 1948. Throughout most of the 20th century the number of nitro musks proliferated and were used extensively in not only perfume but soaps, detergents, and air fresheners.

The time of the nitro musks would come to an end in 1981 because of suspected neurotoxicity. For perfume there was also another reason as these molecules were very sensitive to light and under prolonged exposure to sunlight the nitro musks would decompose. It was probably the decomposition of nitro musks that has led to the concept of perfume “going bad”. As a medicinal chemist when I first looked at these molecules I wasn’t surprised as, by 1981, it was well-known in the pharmaceutical community that these molecules produced significant side effects in clinical trials of molecules that contained them.

Obviously that isn’t the end of the story of musk, just the nitro musks, and in next month’s Olfactory Chemistry I’ll pick up the tale as chemistry reacted and came up with a new class of musk.

Mark Behnke

Olfactory Chemistry: Ionones, Irones, and Methyl Ionones- Purple Rain


Two of my favorite notes in perfumery are violet and iris. The molecules which provide the smell of both of these notes are from a family of closely related molecules called ionones and irones. As you can see below the structures are amazingly similar:


By just adding the CH3 group (in red), what we call a methyl group, to the ring of ionone you get alpha-Irone. If you add the methyl group to the end of the string of atoms to the right of Ionone you get alpha-methyl ionone.

The ionones, irones, and methyl ionones are arguably the second set of synthetic molecules to mark the beginning of modern perfumery. The use of synthetic coumarin in Fougere Royale in 1882 is the acknowledged beginning of modern perfumery. The three molecules above would be isolated in 1893 and have formed the building blocks of many of the synthetic aromamolecules in the 121 years since their discovery.

The olfactory differences in these three molecules are dramatic considering the tiny change in their structure. Alpha-Ionone gives off a woody violet quality with a bit of raspberry to it. Alpha-Irone is the smell of iris. Alpha-Methyl Ionone is softer and imparts the powdery quality to iris. If it was just these three molecules that would be more than enough but the reality is far richer as you can see below by just moving the double bond in the lower half of the molecule you create a new set of molecules called beta or gamma-Ionones and the analogous irones and methyl ionones.


Beta-Ionone has a fruitier and woodier quality compared to the alpha-Ionone. Gamma-ionone is the standard violet you find as an aromachemical.

This all leads to a discussion I had on one of my Facebook perfume groups. One of the members asked about the difference between orris and iris when listed in the perfume notes. The difference is orris is a natural source of irones. Orris is the dried root of iris and contains six of the Irone variations described above. The idea is that the combination of all six naturally occurring Irones present in the root together forms that complex iris effect so prized by perfumers. Because of the need to dry the root for five years after harvest makes orris one of the costliest ingredients in all of perfume. This is why you will usually only see it on the more expensive niche brands note lists. It provides a singular effect but at a high cost. As a result when a perfumer wants a bit of iris character in their fragrance without breaking the budget they will turn to the isolated synthetic molecules like alpha-irone and use that as it is much less costly than orris concrete or orris butter.

In the past thirty years these molecules have been the starting point for the discovery of numerous synthetic molecules which have seen widespread use like Norlimbanol and Iso E Super.

So when you smell a bit of purple in your fragrance it is likely due to one of the molecules related to the ionones, irones, or methyl ionones.

Mark Behnke