Bisabolane, Curcumane, etc

A number of bisabolane sesquiterpenes, including ) -bisabolene (51), iso-bisabolene (52), and cryptomerion (53) have been synthesised by standard procedures. The structure of cryptomerone (54) has been established largely on the basis of n.m.r. spectra. A short and efficient route to nuciferal (55) has been described by Biichi and Wiiest. The same authors have also synthesised bilabanone (56) starting from (-l-)-carvone.  

Two new syntheses of the juvenile hormone, juvabione (57a) have been eompleted. In the first synthesis, i -(-f )-limonene was treated with disiamyl-borane and the adduct oxidised to yield the two alcohols (58a) and (58b) which were separated by fractional crystallisation of their diastereoisomeric 3,5-dinitrobenzoates and subsequent hydrolysis. Eaeh alcohol was separately converted to the corresponding nitrile and these on treatment with isobutyl-lithium gave the optically pure ketones (59a) and (59b). Oxidative modification yielded 

The second synthesis is based on the acid-catalysed fission of the Dieb-Alder adduct (60) to give the 4-substituted cyclohexenone (61). Further elaboration and chromatographic separation yielded the ester (62) (stereochemistry of the isobutyl group unassigned). Reduction of this compound with calcium in liquid ammonia followed by oxidation gave racemic juvabione. 

Hot on the heels of the recent isolation of sesquicarene (63) five independent syntheses have been reported. In essence, all these syntheses have depended upon an intramolecular carbene-olefin cyclisation, viz. of (64, R = Me), (64, R = H), and (65). Only those syntheses which ensured the trans-A double bond in the precursors were stereospecific. Recently, Corey and Achiwa have shown that mercuric iodide not only catalyses the diazo decomposition of (65) (i.e. cis,trans) but also isomerises the A double bond of the trans,trans analogue of (65). Thus, sesquicarene can now be obtained from commercially available farnesol trans,trans cis,trans, 1.5 1) in approximately 35% 

Nakatani and T. Yamanishi, Agric. and Biol. Chem. (Japan), 1969, 33, 1805. 

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