FLEXIBLE MOLECULES WITH DEFINED SHAPE .7. CONFORMATIONAL-ANALYSIS OF 3,5-DIMETHYLHEXENE DERIVATIVES, SUBUNITS OF POLYKETIDE DERIVED NATURAL-PRODUCTS

2,4-Dimethylpentane (1) is a typical biconformational backbone segment . When one methyl group in 1 is replaced by a sp(2) bound residue (vin yl, formyl, phenyl), as in compounds 3-6, a small preference for confo rmer 2b ensues, in which the sp(2) bound group resides in the sterical ly more hindered position. Oxygen substituents in the 3-position do no t change this conformational preference, but in concert with bulkier s p(2)-hybridized groups in the 2-position, a preference for the 2a back bone conformation may be induced, in which the sp(2)-hybridized group is in the less sterically hindered end-of-chain position, cf, compound s 20 and 23.