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

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REACTIVITY OF NITRONATES 591<br />

3.4.4. Reactions of T<strong>and</strong>em [4 + 2][3 + 2] Cycloaddition of Conjugated<br />

Nitroalkenes <strong>and</strong> their Use <strong>in</strong> <strong>Organic</strong> <strong>Synthesis</strong><br />

One of the ma<strong>in</strong> trends <strong>in</strong> modern organic synthesis is the use of efÞcient <strong>and</strong><br />

selective reaction sequences, which make it possible to assemble target molecules<br />

from simple precursors with the use of a m<strong>in</strong>imum number of operations (technological<br />

steps).<br />

In this respect, t<strong>and</strong>em [4 + 2][3 + 2] cycloaddition of nitroalkenes (Chart 3.18),<br />

which was developed <strong>and</strong> studied <strong>in</strong> detail by Prof. Denmark <strong>and</strong> coworkers, is<br />

of most <strong>in</strong>terest for the chemistry of nitronates.<br />

This approach has been comprehensively described <strong>in</strong> Reference 99 <strong>and</strong> two<br />

monographs 427 <strong>and</strong> 428. Hence, we will not consider this approach <strong>in</strong> detail,<br />

the more so that selected aspects of [4 + 2]-cycloaddition reactions of conjugated<br />

nitroalkenes with oleÞns were discussed <strong>in</strong> Section 3.2.1.2.2.2. Many<br />

concerned with the synthesis of six-membered cyclic nitronates, many problems<br />

of [3 + 2]-cycloaddition of six-membered cyclic nitronates were also considered<br />

above (see Sections 3.4.3.1.4 <strong>and</strong> 3.4.3.3).<br />

However, it is necessary to formulate the most common features of the strategy<br />

developed by Prof. Denmark <strong>and</strong> summarize the most important synthetic results.<br />

This strategy is very useful for the synthesis of natural compounds <strong>and</strong> their<br />

analogs. Let us consider this approach <strong>in</strong> more detail us<strong>in</strong>g sequence I (Chart<br />

3.18) as an example.<br />

Initially, after the retrosynthesis of the target substrate, the start<strong>in</strong>g reagents<br />

are chosen (for I, nitrooleÞn <strong>and</strong> two alkenes with opposite dem<strong>and</strong>s). Then<br />

<strong>in</strong>termediate six-membered cyclic nitronate (259) is synthesized (Scheme 3.172).<br />

Here, the difference <strong>in</strong> the rates of [4 + 2]- <strong>and</strong> [3 + 2]-cycloaddition reactions<br />

with respect to the second component (oleÞn) is used. Electron-rich oleÞns<br />

O<br />

A N<br />

O D<br />

I II<br />

III<br />

<strong>in</strong>ter [4+2] / <strong>in</strong>ter [3+2]<br />

O<br />

A<br />

N<br />

O<br />

D<br />

<strong>in</strong>tra [4+2] / <strong>in</strong>ter [3+2]<br />

A – electron acceptor group<br />

D – electron donor group<br />

IV<br />

O<br />

A<br />

<strong>in</strong>ter [4+2] / <strong>in</strong>tra [3+2]<br />

O<br />

A<br />

N<br />

N<br />

O<br />

O<br />

D<br />

D<br />

<strong>in</strong>tra [4+2] / <strong>in</strong>tra [3+2]<br />

Chart 3.18 Various modes of t<strong>and</strong>em [4 + 2][3 + 2] cycloaddition of nitroalkenes.

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