Isoxazoline derivatives Subject

A related strategy was used to prepare D-allosamine (134). Cycloaddition of the dipole derived from nitroacetal (129) to (S )-vinyl dioxolane (74) afforded a mixture of erythro/threo isoxazolines 130 131 (Scheme 6.72). The erythro isoxazoline was subjected to hydroxylation as described above, to give 4-hydroxyisoxazoline 132 with high diastereoselectivity. Lithium aluminium hydride reduction furnished a single diastereomer of aminodiol 133, which could be deprotected to give the hydrochloride salt of D-allosamine (134) (141). 

The sesquiterpene skeleton has also been assembled by the intramolecular nitrile oxide cycloaddition sequence. Oxime 238 (obtained from epoxy silyl ether 237), on treatment with sodium hypochlorite gave isoxazoline 239, which was sequentially hydrolyzed and then subjected to the reductive hydrolysis conditions-cyclization sequence to give the furan derivative 240 (330) (Scheme 6.93). In three additional steps, compound 240 was converted to 241. This structure contains the C11-C21 segment of the furanoterpene ent-242, that could be obtained after several more steps (330). 

This chapter deals mainly with the 1,3-dipolar cycloaddition reactions of three 1,3-dipoles azomethine ylides, nitrile oxides, and nitrones. These three have been relatively well investigated, and examples of external reagent-mediated stereocontrolled cycloadditions of other 1,3-dipoles are quite limited. Both nitrile oxides and nitrones are 1,3-dipoles whose cycloaddition reactions with alkene dipolarophiles produce 2-isoxazolines and isoxazolidines, their dihydro derivatives. These two heterocycles have long been used as intermediates in a variety of synthetic applications because their rich functionality. When subjected to reductive cleavage of the N—O bonds of these heterocycles, for example, important building blocks such as p-hydroxy ketones (aldols), a,p-unsaturated ketones, y-amino alcohols, and so on are produced (7-12). Stereocontrolled and/or enantiocontrolled cycloadditions of nitrones are the most widely developed (6,13). Examples of enantioselective Lewis acid catalyzed 1,3-dipolar cycloadditions are summarized by J0rgensen in Chapter 12 of this book, and will not be discussed further here. 

Bhattacharjya and co-workers reported the synthesis of chiral cyclic ether fused isoxazolines via INOC of allyl glucose derivatives [136]. The pentaallyl nitroalkane 131 when subjected to nitrile oxide generation under Mukaiyama... 

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