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Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

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REACTIONS OF DIPOLAR 1,3-CYCLOADDITION ([3 + 2] CYCLOADDITION) 323<br />

Table 2.20 Isoxazolid<strong>in</strong>es 497 <strong>and</strong> 498 produced via Scheme 2.241<br />

Reaction Trans/cis<br />

Entry R time (h) ratio (497:498) Yield a<br />

a CH2OH 48 62:38 497a, 38%<br />

497a, 20%<br />

b CH2NHBoc 48 72:28 Inseparable<br />

c CH2Br 40 65:35 Inseparable<br />

d CH2SiMe3 40 95:5 497d, 64%<br />

e CH2P(O)(OEt)2 50 74:26 497e, 23%<br />

498e, 4%<br />

a Yield of pure materials obta<strong>in</strong>ed after silica gel chromatography.<br />

R<br />

R<br />

R 2<br />

N+<br />

−<br />

1 O<br />

R 2<br />

R 2<br />

R 1<br />

N<br />

499 500 501 502<br />

+ N −<br />

1 O<br />

499<br />

+<br />

DMPO<br />

N O<br />

+<br />

+<br />

−<br />

+<br />

R 2<br />

R 1<br />

N<br />

O<br />

O<br />

R 2<br />

R 1<br />

R 1<br />

R 2<br />

503 504 505<br />

503<br />

R 1 , R 2 = CH 3, Ph<br />

N O<br />

Scheme 2.242<br />

N<br />

N<br />

O<br />

O<br />

N<br />

506 507<br />

Cycloaddition of allyl alcohol to the D-glucose-derived nitrone (510) is the key<br />

step <strong>in</strong> the synthesis of polyhydroxy <strong>in</strong>dolizid<strong>in</strong>e alkaloids, namely, 2-hydroxy-<br />

1-deoxycastanosperm<strong>in</strong>e (513a,b) <strong>and</strong> 2-hydroxy-1-deoxy-8a-epi-castanosperm<strong>in</strong>e<br />

(513c,d). The <strong>in</strong>termolecular 1,3-cycloaddition of allyl alcohol to nitrone<br />

(510), followed by tosylation afforded the four diastereomeric sugar-substituted<br />

isoxazolid<strong>in</strong>es (511a–d) with the desired regioselectivity. The one-pot conversion<br />

of (511a–d) to pyrrolid<strong>in</strong>es (512a–d) by hydrogenolysis, followed by removal of<br />

O

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