Abstract
The studies on the synthesis of masked o-benzoquinones (MOBs) 99-104 embedded with glucopyranoside as chiral auxiliary to induce chirality in MOB and their subsequent diastereoselective Diels-Alder reaction are described.The in situ generated MOBs 99-104 by the oxidation of chiral phenols 90-95 in the presence of diacetoxyiodobenzene (DAIB) as oxidizing agent underwent diastereoselective Diels-Alder reaction with methyl vinyl ketone to provide cycloadducts 105-110 (a, b, c). Interestingly, the reaction temperature was found to affect the diastereoselectivities obtained. The best diastereoselectivities were obtained when the tandem oxidation/Diels-Alder reaction was carried out at -30 oC with R-group in 90-95 was t-butylbenzyl. However, when the temperature was further lowered to -78 oC, the bulky protecting group made the oxidation difficult and provided inferior diastereoselectivities.A mixture of crude Diels-Alder adducts when subjected to methanolysis under acidic conditions replaced the glucopyranoside chiral auxiliary with methoxy- group and thus provided enantiomerically pure bicyclo[2.2.2]octenone systems 114.The assigned absolute configuration of 105-110 (a, b, c) is based on single crystal X-ray analysis, optical rotation values and NOE data. This reaction was found to be highly facial selective and surprisingly, the dienophile preferred to approach from sterically more hindered face i.e., towards the glucopyranoside auxiliary side.