Menthone photocycloaddition

Asymmetric [2 + 2] photocycloadditions. Acid-catalyzed condensation of t-butyl acetoacetate with (— )-menthone (1) results in two dioxacyclohexenones, 2 and 3, which are easily separable. Photocycloaddition of ( — )-2 with methylcyclo-butene gives mainly the adduct (4) which has the cis, anti, cis-configuration at the... 

Studies of the asymmetric photocycloadditions of the 1,3-dioxacyclohexenones (-)-20 or (-)-23, respectively with methyl cyclobutene opened a new route for the syntheses of enantiomerically pure (—)- and (-i-)-grandisol (19)81,82. This approach adopts the use of stereofacial differentiation by a rigid spirocyclic arrangement of the auxiliary and the enone. Since the alkene adds preferentially to face a of both enones, (— )-20 and (—)-23, Complementary enantiomeric product channels are available operating from a single enantiomer of the auxiliary in each case. The photocycloaddition reactions proceeded in a head-to-head fashion (HH/HT 7 1 at — 78 °C) to give tricyclic products with cis-anti-cts stereochemistry ( + )-21 and (+)-24, respectively. The auxiliary, ( —)-menthone, was smoothly removed by formic acid treatment of the cycloadduct (+)-21, no epimerization of the acetyl group in (— )-22 was observed under these mild conditions. 

In another approach to enantioselective 2 + 2 photocycloadditions, the easily available chiral l,3-dioxin-4-ones have been used as synthetic equivalents of acetylacetates in de Mayo reactions. Chiral information is introduced through a ketalic functional group and menthone derivatives efficiently control the facial selectivity, as indicated in Scheme 30 [155-157]. 

Scheme 30. Enantioselective [2 H- 2] photocycloadditions by using l,3-dioxin-4-ones derived of menthones...

In photocycloadditions of 1,2-dioxine-4-ones A derived from menthone with unsymmetrically substituted alkenes moderate regioselectivity was observed. However, the cycloaddition reaction with cyclopentene proceeded highly stereoselective. " Chiral bicyclic lactams diastereoselectively react with ethylene furnishing optically active cyclobutanes. ... 

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