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X-Ray and lH and 1% n.m.r. data have been obtained for the dibenzocyclo-octadiene lignan kadsurin and several of its synthetic precursors, in which di-ortho-substitution of the aryl rings leads to high energy barriers for rotation around the biaryl bond. The crystallographic and spectral information shows that for compounds in this series the eight-membered ring adopts either a twist-boat (TB) or twist-boat-chair (TBC) conformation. The relative stability of ring conformers and/or biaryl rotamers is determined by three factors : (i) a preference for the TBC over the TB form, (ii) stabilization of the TB conformation by conjugation of the benzylic carbonyl groups with the aryl rings, and (iii) the presence of benzylic hydroxy (or acetoxy) substituents i n endolexo orientations.

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Reactions of l-(phenylsulfonyl)-2-methylene-3-bromopropane (1) with various nitroolefins have been investigated with the purpose of devising a tandem [3 + 21-annulation leading to nitro-substituted derivatives of methylenecyclopentanes. The conjugate addition step occurred readily affording 4a-e (syn) and 5a-e (anti) adducts. The anti-adducts cyclized further in the presence of HMPA to give stereoisomeric methylenecyclopentane derivatives 6a-f and 7a-f. At higher reaction temperature, partial ring closure of 4a-d-syn isomers to give cis-2,3-disubstituted cyclopentanes 8a-d could also be achieved. Base-induced equilibration provided in a one-vessel operation, starting from 1 and 1-nitrobutene (3b), the methylenecyclopentane 6b with three stereogenic centers. The difference in the stereochemical outcome of the Michael addition step with nitroolefins, as compared with reactions in which a,fl-unsaturated esters served as acceptors for 1, may be attributed to the better chelating ability of the lithium counterion in the latter reactions, based upon the results obtained in conjugate additions of methallyl sulfone 9 with 1-nitropropene and crotonic ester. The presence of HPMA strongly influenced the stereochemical outcome of reactions of 9 with ethyl crotonate but not with 1-nitropropene as the Michael acceptor. Reactions of 1 with 1-nitro-2-arylethenes afforded trans-trans-substituted methylenecyclopentanes as the major products. The reaction of 1 with 1-nitrocyclohexene afforded the cis-hydrindane derivative 21. The described reactions provide the first example of intramolecular trapping of nitronates resulting from Michael additions by alkyl/allyl halides.

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