ISOC

Recently, solicon-tethered thastereoselecdve ISOC reactions have been reported, in which effective control of remote acyclic asymmetry can be achieved fEq 8 91) Whereas ISOC occur stereoselecdvely, INOC proceeds v/ith significandy lower levels of diastereoselecdon The reaction pathways presented in Scheme 8 28 suggest a plausible hypothesis for the observed difference of stereocontrol The enhanced selecdvity in reacdons of silyl nitronates may be due to l,3- illylic strain The near-linear geometry of nitnle oxides precludes such differendadng elements fScheme 8 28 ... 

Intramolecular Silyl Nitronate-Olefin Cycloaddition (ISOC)... 

Although nitrile oxide cycloadditions have been extensively investigated, cycloadditions of silyl nitronates, synthetic equivalent of nitrile oxides in their reactions with olefins, have not received similar attention. Since we found that the initial cycloadducts, hl-silyloxyisoxazolidines, are formed with high degree of stereoselectivity and can be easily transformed into isoxazolines upon treatment with acid or TBAF, intramolecular silylnitronate-olefin cycloadditions (ISOC) have emerged as a superior alternative to their corresponding INOC reactions [43]. Furthermore, adaptability of ISOC reactions to one-pot tandem sequences involving 1,4-addition and ISOC as the key steps has recently been demonstrated [44]. 

Entry n R X INOC Yield % Reaction 157 158 ISOC Yield % Reaction 157 158... 

Comparison of INOC and ISOC reactions for the construction of six-mem-bered ethers 157-158J and thioethers 157-158k showed a reversal of stereoselectivity (see entries j and k). 

One-pot tandem sequences involving 1,4-addition and ISOC as the key steps have been developed for the construction of N and 0 heterocycles as well as of carbocycles [44]. In this sequence, the nitronate arising from 1,4-addition to an a, -unsaturated nitro alkene is trapped kinetically using trimethyl silyl chloride (TMSCl). The resulting silyl nitronate underwent a facile intramolecular 1,3-dipolar cycloaddition with the unsaturated tether (e.g.. Schemes 20-22). 

ISOC reaction was employed to synthesize substituted tetrahydrofurans 172 fused to isoxazolines (Scheme 21) [44b]. The silyl nitronates 170 resulted via the nitro ethers 169 from base-mediated Michael addition of allyl alcohols 168 to nitro olefins 167. Cycloaddition of 170 followed by elimination of silanol provided 172. Reactions were conducted in stepwise and one-pot tandem fashion (see Table 16). A terminal olefinic Me substituent increased the rate of cycloaddition (Entry 3), while an internal olefinic Me substituent decreased it (Entry 4). 

Formation of trans isomers in overwhelming predominance in the ISOC reaction leading to five-membered rings (Entries a-d) has been ascribed to the orientation in which H% H , and R are on the exo face of TS 182b (this avoids a possible strain between R and NO or between H and [48b] that is presumably present in TS 182 a). Since elimination of silanol involving H in no way interferes with the orientation of H and R, a trans relationship between H and is abundantly clear. This fully accords with the widely accepted view that approach of the dipole and dipolarophile takes place in two parallel planes [49] and that the endo TS is preferred in the absence of obvious steric effects [50]. Formation of approximately 5% cis isomer when the dipolarophile terminus is disubstituted is accountable in terms of the cycloaddition taking place via TS 182a. 

A strategy involving sequential 1,3-dipolar cycloadditions has been reported for the synthesis of novel bis-isoxazolo substituted piperidines 192a and 192b (Eqs. 18 and 19) [53]. It consists of the Michael addition of an unsaturated alkox-ide 185 to )3-nitrostyrene 184 followed by an INOC or ISOC reaction to provide isoxazolines 187-189 (Eq. 18 and Table 18). A polymer supported acyl chloride... 

Potential precursors to stereoselective INOC and ISOC reactions (e.g., 195 and 196, respectively) have been prepared via stereoselective conjugate additions of several allylic alcohols (e.g., 194,X = 0) and an allylic thiol (e.g., 194,X = S) to a chiral (E)-nitro alkene (e. g., 193) that was derived from (P)-2,3-isopropylidene... 

High levels of diastereocontrol in an ISOC reaction were induced by a stereogenic carbon center that bears a Si substituent (Scheme 23) [55]. For instance, conversion of nitro alkenes (e.g., 199) to j3-siloxyketones (e.g., 203) has been accomplished via a key ISOC reaction-reduction sequence with complete control of 1,5-relative stereochemistry. The generality of the ISOC reaction of a silyl nitronate with a vinylsilane was demonstrated with seven other examples. Corresponding INOC reaction proceeded with lower stereoselectivity. 

Stereocontrolled syntheses of the natural iridolactone, (-l-)-iridomyrmecine 207 (Eq. 21) and the alkaloid (-)-actinidine 213 (Scheme 24) have been accomplished based on an ISOC reaction as the key step [56,57]. ISOC reaction of 204... 

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