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A novel radical induced fragmentation in a benzoxocane ring system has been employed for a synthesis of elvirol 8 and an advanced intermediate in the synthesis of 7,8-dihydroxycalamenene 20. The coumarins 9, 22 were taken through a sequence of reactions to the benzoxocanols 16, 30, which when subjected to Barton’s deoxygenation conditions involving conversion to the corresponding thionocarbonates 17, 31 followed by exposure to tri-n-butyltin hydride, underwent a radical induced fragmentation to furnish elvirol 8 and the curcuphenol derivative 32, which was converted to the dimethoxy compound 33. This had served as an advanced intermediate in a previous synthesis of dihydroxycalamenene.

A convenient approach to the linear tetrahydrofurano benzopyran ring system of xyloketals is described. An orthoester Claisen rearrangement of a chromenol and an intra-molecular cationic cyclization are the key steps in the synthesis. A short, stereocontrolled and high yield synthesis of the phytotoxic metabolite alboatrin was achieved employing this strategy. A unique case of Lewis acid catalyzed isomerization of epi-alboatrin to alboatrin was observed. Subsequently this methodology was extended for the first total synthesis of xyloketal G, where a one pot reaction of three steps viz., acetylation, isomerization and demethylation occurred during acetylation of a mixture of nor-o-methyl xyloketal G and nor-o-methyl epi xyloketal G in presence of AlCl3 to furnish xyloketal G in very good overall yield.

A synthesis of the phytotoxic metabolite alboatrin 1 is described. An intramolecular ketene–olefin cycloaddition followed by an oxidative ring enlargement was employed to generate the tricyclic ring system.

The direct Bargellini condensation of coumarins involving reaction with chloroform and acetone in the presence of aqueous sodium hydroxide furnished o -carboxyvinylphenoxyisobutyric acids in good yields. The utility of this new useful protocol was demonstrated by the transformation of the three diesters 9b , 9f and 9g to the sesquiterpenes helianane 4 , heliannuol A 2 and heliannuol C 3 , respectively.

Synthesis of the allelochemicals heliannuols A and K, and the sesquiterpene helianane is described. A regioselective cleavage of a benzo-fused oxabicyclo(5.1.0)octane system under hydrogenation as well as radical induced condition was developed to generate the basic benzoxocane ring system of these unusual sesquiterpenes.

A synthesis of the methyl ether of an epimer of the alleochemical heliannuol E is described. The route involves indium mediated allylation of a benzopyranone carboxylate and subsequent one carbon degradation to a vinyl group.

The base-induced cleavage of non-enolisable ketones leading to a carboxylic acid derivative and a neutral fragment in which the carbonyl group is replaced by a hydrogen, is referred to as the Haller–Bauer (HB) reaction (Eq. (1)) This C–C bond cleavage reaction was first discovered by Semmler1 in 1906, during his classical studies on the degradation of the monoterpene fenchone, which on treatment with sodamide furnished a cyclopentanecarboxylic acid amide (Eq. (2)). Haller and Bauer followed up on this novel observation and explored its generality employing a variety of relatively simple aliphatic and aromatic ketones.

Photolysis of 3-methoxychromones in methanol furnishes chromones through extrusion of the methoxy function. A conjugate addition of methanol followed by a double methoxy elimination has been proposed to account for the cleavage product.

Photolysis of diazoketone 19 in methanol furnished the cyclobutane carboxylate 20. Photolysis in aqueous sodium bicarbonate-THF afforded the acid 21 in better yield. Reduction of the ester followed by oxidation and a subsequent Wittig reaction furnished ± junionone (3). Photolysis of the diazoketone 24 resulted in a 1:1 mixture of cyclobutane carboxylates 25. Treatment of the corresponding carboxylic acids with hydriodic acid furnished the bicyclic lactone 27, a known precursor of grandisol (1). The diazoketone 28, on photolysis yielded a mixture of cyclobutane carboxylates 29 and 30. Conversion of the trans acid 31 to a methyl ketone followed by oxidative functionalisation of the phenyl group gave the keto ester 36 which was isomerised under acid catalysis to 37, which in optically active form had been a precursor to (-) grandisol. Photolysis of diazoketone 38 afforded the cyclobutane carboxylates 39 and 40 in 2:1 proportion. The corresponding mixture of acids on treatment with methyl lithium and subsequent acetylation furnished the keto acetates 43 and 44, resulting in a formal synthesis of planococcyl acetate (4).

Starting from a seven-membered ring compound, the synthesis of thetricyclic ketone ( 25 ) is described concluding a formal total synthesis of (±) isoclovene, the major sesquiterpene from extensive rearrangement ofcaryophyllene. Enroute, a facile base promoted epimerisation and consequent ketone assisted hydrolysis of ketonitrile (12) under mild conditions was observed.