Carbon suboxide 4+2 cycloaddition reactions

Thiolactams 622 treated with carbon suboxide provide mesoionic compounds 623. Their 1,4-dipolar cycloaddition reaction with highly reactive PTAD gives compounds 624, formed by the cycloaddition followed by extrusion of COS, in quantitative yield (Scheme 100) <1995T6651, 1995H(41)1631>. 

The 2 + 2-cycloaddition reaction of a-alkoxyketene-derived imines yields /l-lactams with quaternary stereogenic centres at C(4).33 The 2 + 2-cycloaddition of chiral aminoketenes with chiral imines yields cis-fi-lactams with the absolute stereochemistry of file C(3) and C(4) positions being controlled by the ketene partner only.34 The 2 + 2-cycloaddition of ketenes with (W)-2-/-butyldihydrooxazole (19) yields predominately the regioisomer (20) from steric control rather than the expected electronic control (Scheme 7).35 The double 2 + 2-cycloaddition reaction between ketenylidenetriphe-nylphosphorane (21) and carbon suboxide (22) produces the bis(ylidic) spirocyclobut-anedione (23) (Scheme 8).36 Semiempirical and ab initio calculations have been used to investigate the Lewis acid-promoted 2 + 2-cycloaddition leading to the formation of jS-lactones.37... 

Azides have cumulative double bonds and they are only a small section of the cumulenes encountered in organic chemistry. Cumulenes are often not stable at room temperature and they are isolated as their cyclic dimers, formed in a click reaction. In this case a [2+2] cycloaddition reaction occurs, and often no catalyst is required. Some of the more exotic cumulenes are matrix isolated at low temperatures. For example, alkyliminopropa-dienones, RN =C=CM3, the mono imides of carbon suboxide, are unstable. However, the neopentyl-, mesityl- and o-t-butylphenyl derivatives can be isolated at room temperature and their nucleophilic reactions provide a wide variety of heterocyclic compounds. ... 

The cycloaddition reactions are subdivided into di-, tri- and oligomerization reactions, [2-1-1]-, [2-1-2]-, [3-1-2]- and [4- -2] cycloaddition reactions and other cycloaddition reactions. The insertion reactions into single bonds are also discussed. The cyclodimerization or cyclotrimerization reactions are special examples of the [2-1-2] and the [2-I-2-I-2] cycloaddition reactions, respectively. The cumulenes vary in their tendency to undergo these reactions. The highly reactive species, such as sulfines, sulfenes, thioketenes, carbon suboxide and some ketenes, are not stable in their monomeric form. Other cumulenes have an intermediate reactivity, i.e. they can be obtained in the monomeric state at room temperature and only heat or added catalysts cause di- or trimerization reactions. In this group, with decreasing order of reactivity, are allenes, phosphorus cumulenes, isocyanates, carbodiimides and isothiocyanates. 

The polymerization of carbon suboxide can theoretically be initiated by a ketene like dimerization of the molecule. However, it was shown that the polymerization is induced by trace amounts of nucleophiles, such as water, to give pyrone derivatives, which react with more carbon suboxide to form poly(a-pyrone). Although most reactions of carbon suboxide seem to be initiated by nucleophilic reactions, some cycloaddition reactions of carbon suboxide are known (see Section 3.1.2.2). 

The reaction of carbon suboxide with azomethines 46 was previously believed to involve a double [2+2] cycloaddition however it was subsequently shown that the reaction product is a 1 1 adduct 47 with a switter ionic structure . 

Another [2+2] cycloaddition reaction of carbon suboxide is encountered in the double Wittig reaction with the methylene phosphorane 48, which affords the 1,2,3,4-pentatetraene derivative 49 in 46 % yield . ... 

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