Photochemical cycloadditions, chiral auxiliaries

Some efforts have also been done to induce chirality in the intramolecular [2 + 2] photocycloaddition of benzene derivatives using a chiral auxiliary [88]. 141b was isolated with a diastereoselectivity of 17% from the intramolecular photocycloaddition of the salicylic acid derivative 140b (Scheme 35) [89]. After an initial [2 + 2] cycloaddition, reversible thermal and photochemical rearrangements took place. These equilibria can be displaced by an acid catalyzed and irreversible addition of methanol to the intermediate P. 

Control of absolute asymmetry is a relatively untouched area for [2 + 2] photochemical cycloaddition reactions despite the recent advances in the field of asymmetric synthesis. The first example of the use of a removable chiral auxiliary was reported by Tolbert, who obtained impressive enantioselectivity in the photocycloaddition of bomyl fumarate to stilbenes (equation 37). More recently, Lange has shown that menthyl cyclohexenonecarboxylates are useful in control of absolute stereochemistry (equation 38). Baldwin and Meyers have also obtained excellent facial selectivity in systems where the stereogenic center which controls the diastereoselectivity can be excised to afford products of high enantiomeric purity (equations 39,40). 

Similarly, attempts to make enantioselective the intramolecular cycloaddition of dienes catalyzed by Cu(I), employing a removable chiral auxiliary, were described. The stereochemistry of the major diastereoisomer 86, produced during the photochemical step of 85, could be deduced from a comparison of the diaste-reoisomeric transition states as shown in Scheme 20 [62]. 

A preliminary examination of asymmetric induction in photochemical [2 + 2] cycloaddition reaction of an enone to an alkene employed a chiral auxiliary attached to the alkene component74. The photocycloaddition of 2-cyclopentenone to the optically active ketene acetal 1 led both to oxetanes and cyclobutanes in the ratio 6.5 3.5 with a total yield of 60%. The m-[2C + 2C] addition of the chiral alkene I to 2-cyclopentenone was completely regioselective and gave four diastereomeric head-to-tail cycloadducts 2-4 in the ratio 6 29 33 32. 

A comparison between single and double induction in the photochemical [2 + 2] cycloadditions of 3-oxo-l-cyclohexen-l-carboxylates with the ketene acetals was investigated89 90. Single induction using (-)-menthol as chiral auxiliary [R1 or R2 = (-)-menthyl] gave low selectivities only. A combination of these single inductions [R1 = R2 = (-)-menthyl] leads to the intermediacy of a matched reaction pair which gave an increased diastereoselectivity (d.r. 27.5 72.5) whereas a mismatched" reaction pair [R1 = ( + )-menthyl, R2 = (-)-menthyl] resulted in a decrease (d.r. 41.5 58.5). A more pronounced contrast in double induction results when the two chiral auxiliaries arc not equivalent [R1 = (—)-8-phenylmenthyl, R2 = (—)-menthyl]. 

Photochemical [2 + 2] Cycloadditions. Photochemical [2 + 2] cycloadditions between alkenes and chiral phenylgly-oxylate derivatives of 3-hydroxyisobomeol show minimal diastereoselectivity (16% de). Better results are obtained in [2 + 2] cycloadditions between chiral enol ethers and Dichloroketene (eq 7). After ring expansion and expulsion of the auxiliary (Diazomethane, Chromium ll) Perchlorate),... 

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