H.P.S. Chawla

Semihydrogenation of acetylenes : Modified lindlar catalyst

Abstract The effect of doping Lindlar catalyst with different metal salts (shown in Table 1) on selectivity, during semihydrogenation of some acetylenes to the corresponding olefins, has been studied. Lindlar catalyst modified with MnCl 2 has been found to be more selective and reproducible.

Higher Isoprenoids-XIIa: Partial syntheses from cycloartenol, cyclolaudenol-part 4: a novel method for functionalization of C-4 methyl in triterpenoids, and synthesis of cyclodeucalanone

Abstract A general method for selective oxygenation of C-4-Me in triterpenes, leading finally to 4α-carboxyl/4α-hydroxymethyl functionalities, is described. The key-step involved is phytolysis of hypoiodite derived from 3β-hydroxymethyl derivative of the triterpene. The method is illustrated by the conversion of cyclolaudanone ( 1 ) into methyl 3-oxo-cyclolaudan-29-oate ( 2 ). The latter has been converted into the known cycloeucalanone ( 3 ) by a simple sequence of reactions.

Higher isoprenoids-XI: Partial syntheses from cycloartenol, cyclolaudenol-Part 3: Cyclobuxophyllinine-m, cyclobuxophylline-k and buxanine-m

Abstract 3β-Acetoxy-9,19-cyclo-4,4,14α-trimethyl-5α-pregnan-20-one, an intermediate readily accessible from cycloartenol/cyclolaudenol has been transformed into cyclobuxophyllinine-M, an alkaloid isolated from certain Buxus Spp. This compound has been earlier converted into the closely related alkaloids cyclobuxophilline-K and buxanine-M. The present work formally constitutes the synthesis of these alkaloids from cycloartenol.

Heterolytic cleavage of homoallylic alcohols—I: fragmentation of 6-hydroxycamphene derivatives

Heterolytic fragmentation of homoallylic alcohols incorporated in a bicyclo[2,2,1]heptane system with electrophilic reagents or that of the corresponding epoxides with acids, is described. A short route, via this fragmentation, to synthon 25, potentially useful in the syntheses of cis-jasamone and prostanoids, is reported.

Rearrangement of 3,3-ethylenedioxy-isobornyl tosylate: an efficient new synthesis of 7-ketocamphene

A new efficient route to 7-ketocamphene, involving as the key-step, a solvolytic-type rearrangement of 3,3-ethylenedioxyisoborneol (via tosylate) is described. A side-reaction arising from acid-catalyzed fragmentation of 7,7-ethylenedioxycamphene was encountered and has been investigated further. A Baeyer-Villiger type oxidation with dioxane hydroperoxides in the presence of an acid has been noted.

Steric diversion—I : Addition of halogens and pseudo-halogens to isolongifolene

Abstract Addition of halogens (Cl2, Br2) and pseudo-halogens (ICl, halogen azides, NOCl) to solongifolene does not yield any “normal” addition products due to severe steric hindrance to the approach of counter ion at C-7. Initially formed halogenonium ions undergo elimination or rearrangement to give “abnormal” products. The term Steric Diversion is suggested to describe this switch over from the “normal” route.

A novel method for functionalization of C-4 methyl in triterpenoids. A synthesis of cycloeucalanone

Abstract Functionalization of 4α-methyl group in cycloartane-type triterpenoids has been accomplished utilizing, in the key step, photolytic decomposition of an hypoiodite derived from a 3β-hydroxymethyl-4,4-dimethyl precursor. The synthesis of cycloeucalanone from cyclolaudanone is reported.

Studies in sesquiterpenes—LVII: Isolongifolene (part 8): solvolysis of 8-bromoneoisolongifolene

Abstract Acetolysis of 8-bromoneoisolongifolene generates, besides two normal products (of elimination and SN2 displacement), which were minor, one rearranged elimination product and a tertiary acetate. The rearranged diolefine arises from acetic acid-catalysed rearrangement of the normal elimination product, while the tertiary acetate is a Wagner substitution product arising from a syn-migration. In this context various factors controlling such migrations are discussed and, the possible role of energetics of the incipient carbonium ion at the migration origin highlighted.