Selective hydroboration of a 1,3,7‐triene: homogeraniol intermediate: geranial solvent: 200 mL of petroleum ether intermediate: 13.0 g (86.7 mmol) of (E)‐4,8‐dimethyl‐1,3,7‐nonatriene product: homogeraniol intermediate: 1–2% of the Z isomer.

Dammarenediols I (1a) and II (1b) were prepared by an efficient nonenzymatic biomimetic polyene tetracyclization route. The cyclization substrate, pentaenol 3, contains a tetramethylallylic alcohol initiator, an allyltrimethylsilane terminating group, and a fluorine atom at pro-C-13 to serve as a cation-stabilizing (C-S) auxiliary controlling the regiochemistry of the C/D ring juncture. The synthesis of 3 employed lithium−halogen exchange to create alcohols 10 and 19. The Z-fluoroalkene in 3 was introduced stereoselectively via the Trost palladium-catalyzed alkylation of allylic acetate 11 (Z/E: 4.6/1). The cyclization of 3 was most efficient (62% isolated yield) when it was added as a dilute solution in dichloromethane to trifluoroacetic acid at −45 °C to afford tetracyclic fluoro diene 24 possessing the trans−anti−trans−anti−trans ring stereochemistry of the dammaranes. Replacement of the fluorine atom of 24 with hydrogen with complete retention of configuration was accomplished using the Ohsawa−Oishi reagent (Na/K alloy and crown ether). Wacker oxidation of the resulting hydrocarbon provided ketone 28, which after ketalization was ozonolyzed with a reductive workup to give the 3β-alcohol 30. Ketal hydrolysis followed by Grignard reaction with isopentenylmagnesium bromide afforded the dammarenediols (1/3, 1a/1b).

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