Oxetanes stereocontrol

Stereopure epoxide 1 was prepared and treated with 3.6 equivalents of t-butyllithium in THF/HMPA at -78°C. The intention was that formation of the anion at the benzylic carbon would lead to a 4-exo-epoxide ring opening reaction a subsequent [l,2]-silyl shift (Brook rearrangement) would generate the oxetane 2 with stereocontrol at all three stereocentres. Anion formation proceeded smoothly at -78°C, then 1 ml of 1 M hydrochloric acid was added and the product isolated. Obtained pure in 40% yield, this was shown to be the aldehyde 3. No oxetane 2 was obtained. 

This photochemical umpolung is useful from the perspective of chemoselectivity in synthetic transformations. In addition, oxetane-containing products, prepared regioselectively, can be valuable synthetic intermediates. Both oxetane and vinyloxetane derivatives react with nucleophiles in intermolecular and intramolecular condensations and with transition metals and carbenoids. A second feature of oxetane-containing photoproducts, which has important implications, is that they can act as vehicles for subsequent manipulations and stereocontrolled synthesis. For example, the oxetane photoproduct (6)... 

The synthesis of oxetanes from alkenic precursors has been demonstrated and the range and scope of Patemo-Biichi photocycloadditions are broad. In general, both the reaction regioselectivity and stereoselectivity can often be predicted by considering the reaction mechanism (Section 2.4.2) for example, the directing effects of alkene substituents are readily imderstood. Synthetic applications are numerous owing to the rtq>id stereocontrolled assembly of multifimctional targets. 

Griesbeck, A. G., Bondock, S. Oxetane formation stereocontrol. CRC Handbook of Organic Photochemistry and Photobiology (2nd Edition) 2004, 59/51-59/19. 

Further work has been carried out to study the facial diastereoselectivity of the addition of benzaldehyde to alkenes. ° In this study the three enamines (49)-(51) were used as the substrates to which benzaldehyde was added photochemically. The results show that addition does occur to all three enamines but with varying degrees of success as far as diastereoselectivity is concerned. Thus addition to (49) gives the two oxetanes (52) and (53) but with only 32% de. Poorer selectivity is observed with (50) when (54) and (55) are obtained. The best de of 62% is achieved from (51) where the products are (56) and (57). The photochemical addition of the aldehydoester (58) to the enamine (59) results in the formation of the oxetane (60). This product is obtained in around 30% yield and it can be transformed into racemic oxetin (61). Bach has reviewed the stereocontrol that can be exercised on the formation of oxetanes. The regioselectivity of the addition of triplet carbonyl compounds to alkenes has been interpreted for the first time in terms of hard and soft acid-base systems. The authors of this report suggest that there is overall good agreement between HSAB prediction and experimental fact. 

Stereocontrol in the Paterno-Biichi photocyclization of benzaldehyde and styrenes has been reviewed <97LAl627>. Trimethyl silylether substituents on the styrene were found to give high stereoselectivity in oxetane formation <97TL5407, 97LAI529>. The 3-silyloxy oxetanes have been utilised in the formation of diastereomerically pure 3-alkoxy- and... 

---