Intramolecular nitrile oxide-olefin cycloaddition

Scheme 17. Construction of aglycon 9 intramolecular nitrile oxide-olefin cycloaddition of intermediate 31.

Certain specific steric effects are operative on intramolecular nitrile oxide— olefin cycloadditions. These effects are governed by both ring size and character of substituents. Thus, cycloadditions to the exomethylene group are successful with substituted methylenecyclohexanones 334 (m = 1, 2 n = 2) and gave tricyclic 335 (m = 1, 2), but do not occur with methylenecyclopentanones 334 (m = 1, 2, 3 n = 1). Activation energies calculated by molecular mechanics are consistent with these results. Cleavage of 335 (m = 2) by Raney Ni gives cA-decalone 336 (403). 

Intramolecular nitrile oxide—olefin cycloaddition of oxazolidine and thiazoli-dine oximes 407 (R = H, Me R1 =H, Me X = 0, S n = 1,2) proceed stereose-lectively, yielding tricyclic fused pyrrolidines and piperidines. Thus, 407 (n =2 R = H R1 =Me X=S) has been oxidized to the nitrile oxides with sodium hypochlorite, in the presence of triethylamine in methylene chloride, to give the isoxazolothiazolopyridine 408 in 68% yield. Reduction of 408 with lithium aluminum hydride affords mercaptomethylmethylpiperidine 409 in 24% yield (448). 

A total synthesis of the sesquiterpene ( )-illudin C 420 has been described. The tricyclic ring system of the natural product is readily quickly assembled from cyclopropane and cyclopentene precursors via a novel oxime dianion coupling reaction and a subsequent intramolecular nitrile oxide—olefin cycloaddition (463). 

The formation of compound (16) is proposed to proceed through the nitrile oxide (15) as an intermediate to undergo intramolecular nitrile oxide-olefin cycloaddition (INOC). On the contrary, the generation of (18) is proposed to proceed through intramolecular alkoxycarbonyl nitronate-olehn cycloaddition (lAOC) step because only the trans isomers are formed. 

To conclude this nonexhaustive review of carbocyclization methods, we should mention intramolecular cycloaddition as a useful tool to construct carbocycles. Nitrile oxide-olefin cycloaddition seems to be a useful strategy to constmct carbocycles from carbohydrates because... 

The ability of the intramolecular nitrile oxide-olefin [3 + 2] cycloaddition reaction to furnish isoxazolines suitable for further elaboration is illustrated in Scheme 34 utilizing (Ry mandelic acid (2). Chiral oxime 142, prepared in four steps from 2, is oxidized to a nitrile... 

Chapter Synthesis of 12- to 16-Membered-Ring Lactones is dedicated to the synthesis of 12- to 16-membered ring lactones. In this chapter, M. Kalesse and M. Cordes present an overview of the macrocyclization of seco-acids as well as new effective procedures to access 12- to 16-membered ring lactones such as ringclosing metatheses of alkynes and olefins. The authors also report the use of ketene sources and benzodioxinones to produce macrocyclic lactones. Nitrile oxide-olefin cycloaddition, intramolecular C-H oxidative macrolactonization, and Yamaguchi and Mukaiyama macrocyclization as well as macrolactonization via thioester or using phosphorus reagents are described. 

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]. 

Intramolecular nitrone cycloadditions often require higher temperatures as nitrones react more sluggishly with alkenes than do nitrile oxides and the products contain a substituent on nitrogen which may not be desirable. Conspicuously absent among various nitrones employed earlier have been NH nitrones, which are tautomers of the more stable oximes. However, Grigg et al. [58 a] and Padwa and Norman [58b] have demonstrated that under certain conditions oximes can undergo addition to electron deficient olefins as Michael acceptors, followed by cycloadditions to multiple bonds. We found that intramolecular oxime-olefin cycloaddition (lOOC) can occur thermally via an H-nitrone and lead to stereospecific introduction of two or more stereocenters. This is an excellent procedure for the stereoselective introduction of amino alcohol functionality via N-0 bond cleavage. 

A number of intramolecular cycloadditions of alkene-tethered nitrile oxides, where the double bond forms part of a ring, have been used for the synthesis of fused carbocyclic structures (18,74,266-271). The cycloadditions afford the cis-fused bicyclic products, and this stereochemical outcome does not depend on the substituents on the alkene or on the carbon chain. When cyclic olefins were used, the configuration of the products found could be rationalized in terms of the transition states described in Scheme 6.49 (18,74,266-271). In the transition state leading to the cis-fused heterocycle, the dipole is more easily aligned with the dipolarophile if the nitrile oxide adds to the face of the cycloolefin in which the tethering chain resides. In the trans transition state, considerable nonbonded interactions and strain would have to be overcome in order to achieve good parallel alignment of the dipole and dipolarophile (74,266). 

Nitrile oxide J -I- 2 cycloaddition.1 A key step in a recent stereospecific synthesis of biotin (6) from cycloheptene (1) is an intramolecular [3 + 2]cyclo-addition of a nitrile oxide (a), obtained by dehydration of a primary nitro compound (3), preferably with phenyl isocyanate. This cycloaddilion is more efficient than the well-known olefinic nitrone cycloaddition. The carbon atoms in 6 derived from cycloheptene are marked with asterisks. 

Intramolecular cycloadditions of alkenyl-substituted nitrile oxides produce bicyclic isoxazolines. When monocyclic olefins are used, tricyclic structures are obtained. This approach was pioneered by both Kozikowski s and Curran s groups. A typical case involves the cycloaddition of nitro compound 191 [mixture of diastereomers derived from pentenose pyranoside 190], which produced a diaster-eomeric mixture of isoxazolines that contain cis-fused rings (i.e., 192) in near quantitative yield (326) (Scheme 6.85). Further elaboration of this mixture led to epoxycyclopentano-isoxazoline 193, which was then converted to the aldol product in the usual manner. The hydrogenation proceeded well only when rhodium on alumina was used as the catalyst, giving the required (3-hydroxyketone 194. This... 

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