Hetero Diels-Alder Reactions of Nitroso Alkenes with Alkoxyallene Derivatives Bearing Carbohydrate Auxiliaries: Asymmetric Synthesis of 6H-1,2-Oxazines and Subsequent Reductive Transformations
of the 2-substituted or 2,5-disubstituted dihydropyrrole derivatives could be isolated in diastereomerically pure form and were subsequently converted into the expected pyrrolidin-3-ones by removal of the carbohydrate-derived auxiliary under acidic conditions. The desired products were obtained in good yield and with high enantiopurity. They are suitable starting materials for the synthesis of enantiopure
Syntheses of Enantiomerically Pure 2-Substituted Pyrrolidin-3-onesvia Lithiated Alkoxyallenes - An Auxiliary-Based Synthesis of both Enantiomers of the Antibiotic Anisomycin
The hydrochlorides of both enantiomers of the antibiotic anisomycin were prepared starting with the ‘diacetone-fructose’-substituted allene 1 and the N-Boc-protected imine precursor 2a. Addition of an excess of lithiated 1 to 2a provided a 2 : 1 mixture 3a of diastereoisomers, which were cyclized to 4a under base promotion (Scheme 2). The two diastereoisomers of 4a were separated and converted into
A series of enantiopure 1-alkoxyallenes 1a-1f was prepared starting from propargyl bromide and the corresponding optically active alcohols via propargyl ethers 3a-3f as intermediates. In addition, disubstituted enantiopure allene derivatives 5a-5c were synthesized by isomerization of the corresponding alkynes 4a-4c. Allene derivative 5a was also prepared via an alternative route with 1-trimethylsilylallene derivative 6a as crucial intermediate.
Hetero Diels-Alder Reactions of Nitroso Alkenes with Alkoxyallene Derivatives Bearing Carbohydrate Auxiliaries: Asymmetric Synthesis of 6<i>H</i>-1,2-Oxazines and Subsequent Reductive Transformations
Alkoxyallene derivatives 1a-f bearing carbohydrate auxiliaries at the oxygen were examined in asymmetric hetero Diels-Alder reactions with nitroso alkenes. Diacetoneglucose derived compound 1a turned out to be the best precursor furnishing the primary cycloadducts 3a-c with a diastereomeric ratio of approximately 90:10. Isomerization provided the thermodynamically more stable 6H-1,2-oxazines 4a-c. Similarly, diacetonefructose derived allene 1f gave compounds 4h-j with good efficiency. Gratifyingly, it turned out that 1a and 1f were complementary with respect to the preferential absolute configuration at C-6 of 6H-1,2-oxazines 4a-c and 4h-j, respectively. Cycloadducts derived from 1a have 6S configuration in excess whereas those derived from 1f are predominantly 6R configured. Exhaustive hydrogenolysis of 6H-1,2-oxazines 4a and 4h in the presence of palladium on charcoal furnished the expected primary amine 5 in an enantioenriched form. If this reduction was performed under addition of hydrochloric acid, pyrrolidine derivative 6 together with secondary amine 7 as side product were isolated.