Stereoselective total synthesis of (+-)-presqualene alcohol

Abstract

The possibility of a puckered cyclobutyl cation intervening between 14 and 15 cannot be ruled out:16a C. D. Poulter and S. Winstein, ibid., 91, 3650 (1969).(22) Hydride attack of 16 at C1 should proceed with inversion,12,168 giving the wrong absolute configuration. We have preliminary evidence that a delicate balance exists between vinyl-substituted cyclopropylcarbinyl cations similar to 14 and 15 and their allylic isomers.Thus, the rearrangement 14 --15 --t 16 is quite plausible: C. D.Poulter and S. Moesinger, unpublished results.

Biosynthesis of Terpenoids: The Oldest Natural Products

“…As discussed in the sequel, squalene (C 30 H 50 ) is biosynthesized in a multistep pathway (Fig. 5.20 [40] and two other groups [42,43]. Squalene (6) is a highly branched (tentacled with methyls) symmetrical hydrocarbon having a C 2 -axis.…”

“…Real S N 2 type reactions are rare in biological systems. Rapid stereospecific deprotonation {ÀH A (ÀH R )} gives PSPP (2), the structure and stereochemistry of which has been confirmed [38][39][40][41][42][43] by degradation and an unambiguous synthesis of its alcohol. Since one of the allylic methylene protons is stereospecifically eliminated, the deprotonation should be a fast process, being brought about by a base of appropriate strength.…”

Biogenetic-Type Rearrangements of Terpenes

“…The two perhydrochrysene based triterpenes baccharis oxide (391) (483,484) and shionone (392) are biogenetically derived by backbone rearrangements similar to those portrayed above for glutinol (373) and friedelin (375), but originating from tetracyclic carbonium ion (346) prior to E ring closure (Scheme 30). If cyclization to the bridged oxide moiety of (391) immediately follows the C10 -+-Cg methyl rearrangement, the oxygen nucleophile enters syn to the departing methyl group.…”

“…Once the structure and stereochemistry of the derived alcohol were established as 113b-OH through both chemical degradation (170,171,373) and total synthesis (374)(375)(376), the biogenesis ofsqualene, at least in general terms, became clear (170,373,(376)(377)(378)) (Scheme 27). As we have seen (Section B.l and C.4), the structure of presqualene…”

“…Taraxerol (372) Multiflorenol (375) Friede !in 195 Owing to the greater thermodynamic stability ofthe oleanene skeleton, most of the chemically-induced backbone rearrangements in this series proceed in the opposite direction from the biogenesis. Taraxaryl acetate (371-0Ac) (459), various taraxarane derivatives (460)(461)(462), and multiftorenyl acetate (372-0Ac) ( 463) are readily isomerized to ß-amyrin acetate (352-0Ac), or the corresponding N 2 -oleanene.…”