8.9- Seco-5-lactaranolides

With respect to the aliphatic moiety (cyclopentene ring, plus protons at C-3, C-4, and C-12), 8,9-secofuranolactaranes gave NMR spectra resembling those of 8,9-seco-5-lactaranolide sesquiterpenes (Part 14) in addition, the two protons on the furan ring exhibited the characteristic couple of signals at 57.20-7.36. Remarkably, when a carbonyl group is attached to C-7, as in compounds 19.1 and 19.3, the signal of H-13 is shifted downfield to 57.95. 

Scheme 18 is the Me2AlCl catalyzed ene cyclisation of 8,9-seco-furanolactarane and 8,9-seco-5-lactaranolide sesquiterpenes exemplified by compound 14.1 to the corresponding lactarane sesquiterpenes such as 11.10 98). Lactone (11.10) exhibited the unusual cis configuration between H-8 and H-9. The emergence of this stereorelationship could be anticipated by examining the Dreiding models of the two possible transition states 25.5 and 25.6 (Scheme 19). In fact, unfavourable steric interactions developing between the C-3 methyl group and the bulky >C=0 -Al- complex are minimized in the transition state 25.6, which eventually collapses to lactone 11.10 98).

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