Guaianolides, rearrangement

From six-membered rings Rearrangement as part of the pseudo-guaianolide to confertin synthesis [14]. 

A review has recently appeared describing the synthesis of hydroazulene sesquiterpenes through rearrangement of substituted hydronaphthalene precursors, including some syntheses of guaianolides from santonin (1)... 

Transformation of santonin (1) into guaianolides has been carried out by two different methods, namely photochemical rearrangement of the dienone system present in the A ring of santonin, and solvolytic rearrangement of a proper 1-sulfonate derivative. 

The overall yield of this synthesis was poor and so the same authors devised a new sequence starting from compound 206, obtained from santonin in 10 steps in 23% overall yield (Scheme 27). Solvolysis of 206 gave, in 75% yield, a 2 1 3 mixture of guaianolides 207, 208 and 209, which possess tetra-, tri-, and di-substituted double bonds, respectively. It is likely that the solvolytic rearrangement of206 proceeded via 210, since... 

Stereoselective rearrangement of guaianolides bearing a double bond at the C(6)/C(6a) position occurs to give tricyclic S-valerolactones (Scheme 44). ... 

FIGURE 20.76 Biotransformation of (-)-dehydrocostuslactone (249) and rearranged guaianolide (252a) by... 

A early partial synthesis of the guaianolide nucleus was based on photochemical rearrangement of a-santonin (44) to give isophotosantonic lactone (45) [17] (Scheme 98.5). The stereochemistry of this reaction is remarkable. Irradiation of the santonin isomer, in which the C6-06 bond is 3 disposed gives the C6-06, C7-C11 cis isomer of 45 [18]. 

Bohlmann, F., J. Jakupovic, R.M. King, and H. Robinson New germacrano-lides, guaianolides and rearranged guaianolides from Lasiolaena santosii, Phytochem. 20, 1613 (1981). 

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