Selective Degradation of Grindelic Acid and Its 3α-Hydroxy Analog: Facile Synthesis of Grindelistrictic Acids and Potential Intermediates for Erigerol

Facile synthesis of grindelistrictic acid (2a; 5,5,5′-8a-tetramethyl-3-oxo-3,4,4′ ,4a,5,5′ ,6,7,8,8a-decahydrospiro [1H-2-benzopyran-1,2′(3′H)-furan]-5′-acetic acid) and its 3α-hydroxy analog (2b) from grindelic acid (1a) and 3α-hydroxygrindelic acid (1b) was attained by selective degradation via oxidative cleavage of the 7-acetyl-7-hydroxy derivatives (11a,c; methyl 2′-acetyl-2′-hydroxy-2′, 3′, 3′a, 4,4′, 5,5′, 6′, 7′, 7′a-decahydrospiro[furan-2(3H),1′-[1H]indene]-5-acetates) in high yields. 3α-Acetoxy-13,13-ethylenedioxy-6β,9β-dihydroxy-7,8β-epoxy-15, 16-dinor-ent-labdane (21), a potential intermediate for synthesis of erigerol (3), was derived from 1b via S_N 2′ reaction of methyl 3α-acetoxygrindelate (6c) with thiophenol followed by retro-aldol degradation of the 7β-phenylthio derivative (14).