Isopropylidene function

A new method of constructing isopropylidene functionality has been illustrated recently in the synthesis of 4(14),7(ll)-selinadiene (202), a eudesmane derivative present in hops.92 Interest continues in the isolation, structural elucidation, and... 

The selective hydrolysis of acetyl groups, led in methanol in presence of concentrated sulphuric acid, gives the a,P unsaturated methyl ester 13e. Therefore the two hydroxyls at C-3 and at C-4, in sin position each other, are protected by means of formation of isopropylidene function, doing react the methyl ester 13e with 2,2-dimetiioxypropane in acetone in presence of catalj tic quantity of strongly acidic resin, so obtain the 3,4-O-isopropylidene derivative 13f. 

Since the isopropylidene function blocks the two hydroxyl groups these are in position 3 and 4 in sin of quinic acid, the double bond position must be 1,2 and not 1,6. 

To explain this regioselectivity there is not a certain reason probably the hydroxyls in position 3 and 5 have a important role in the dehydration process. In fact hydroxyl at C-3 has an absolute configuration which corresponds to a equatorial position, while hydroxyl at C-5 has an absolute configuration which corresponds to an axial position. Obviously the transition state, which corresponds to the dehydration that brings to the product 13d, is more energetically favoured than that one that should bring to the other isomer. Then the isopropylidene function in 13g is selectively eliminated by means of an acidic hydrolysis led in a solution of 80% acetic acid at 70°C, to obtain derivative 13h in which the hydroxyl at C-5 is selectively protected. 

Further examination of the extracts of A. cannabina revealed axisonitrile-4 (7), axisothiocyanate-4 (8) and axamide-4 (9) [33], A vinylic isonitrile function was supported by H NMR signals at <51.67 and 1.89, which were assigned to the two isopropylidene methyls of 7. Difficulty in isolating the natural product 9 was circumvented, when isonitrile 7 was transformed to 9, mp 81-84 °C, by acetic acid in anhydrous ether. The absolute configurations of both axanes 1 and 7 and their analogs were later established [31] by studies including X-ray diffraction of the p-bromoaniline derivative of 2 and by CD data of ( + )-10-methyldecalone-l obtained from ozonolysis of the reduction (Na/NH3) product of 1 [1]. 

Reaction of D-glucono-1,4-lactone with 2,2-dimethoxypropane-tin(II) chloride yields the 5,6-0-isopropylidene derivative 13, which on periodate oxidation afforded 2,3-0-isopropylidene-D-glyceraldehyde (21). However, the acid-catalyzed isopropylidenation of D-glucono-1,5-lactone with 2,2-dimethoxypropane afforded methyl 3,4 5,6-di-0-isopropylidene-D-gluco-nate (14) as the main product (22). Reduction of the ester function gave... 

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