Cycloaddition reactions regiochemistry

Bis(tnfluoromethyl)-4,5-dihydrooxazin-6-ones [28] and their O-acetylated dcnvatives [96] are formed on treatment of acyl imines with acetyl chloride-hiethylamine at room temperature. The reaction was interpreted as a cycloaddition reaction involving a ketene [28] However, the periselectivity and regiochemistry of this reactwn-are not in agreement with results obtained from the reaction of... 

The A-benzenesulfonyl imines of hexafluoroacetone readily react with nitrile oxides to give [3-1-2] adducts, apparently in a multistep reaction [151] (equation 36) Although only a few examples of [3-1-2] cycloaddition reactions of this type have been descnbed so far, most 1,3-dipoles should react in this way with predictable regiochemistry [5 146, ISO 151]... 

In contrast, when ot,P-unsaturated multiple bond systems act as dienophiles in concerted [4+2] cycloaddition reactions, they react across the C=C double bond Periselectivity as well as regiochemistry are explained on the basis of the size of the orbital coefficients and the resonance integrals [25S]... 

An interpretation based on frontier molecular orbital theory of the regiochemistry of Diels Alder and 1,3-dipolar cycloaddition reactions of the triazepine 3 is available.343 2,4,6-Trimethyl-benzonitrile oxide, for example, yields initially the adduct 6.344... 

The [3 + 2]-cycloaddition reactions of allenes with 1,3-dipoles are useful for the construction of a variety of five-membered heterocycles with a high degree of regio- and stereochemical control [67]. Generally, the dipolar cycloaddition reactions are concerted and synchronous processes with a relatively early transition state. The stereoselectivities and regiochemistries are accounted for by the FMO theory The reaction pathway is favored when maximal HOMO-LUMO overlap is achieved. 

Vollhardt and colleagues338b studied the regiochemistry in these cycloaddition reactions. When the a,cu-diynes had large substituents at both termini, the reaction with W-phenylsulfonylindole did not afford any adduct due to steric hindrance. When smaller substituents were present, the cycloaddition proceeded in such a way that the larger substituent was distant from the phenylsulfonamide moiety, as illustrated for the reaction of 585 with 586 (equation 168). Anti 587 and syn 588 were obtained in a 61 39 ratio. 

In certain cases, 4,5-dihydro-1,2,3-diazaphospholes rather than 3,5-dihydro-1,2,4-diazaphospholes are formed from the [3-1-2] cycloaddition reaction of diazo compounds with phosphaalkenes. This regiochemistry was encountered in the reaction of (mesityl)P=CPh2 with diazodiphenylmethane and was attributed to steric factors (164). Electronic factors may explain the orientation found in the... 

The reaction of nitrile oxides with 4-arylmethylene-5(4//)-oxazolones 675 to give the corresponding spiroisoxazoline oxazolones 676 is also well known.The regiochemistry of this cycloaddition reaction was initially incorrectly assigned but a careful study of the reaction showed that the regiochemistry of the 1,3-dipolar cycloaddition of nitrile oxides is the same as that observed with nitrile imines (Scheme 7.213). Examples of spiroisoxazoline oxazolones are shown in Table 7.49 (Fig. 7.60). 

The regiochemistry of ketene iminium salt cycloadditions can also differ from ketene cycloadditions. Whereas reaction of styrene with a series of ketene iminium salts gave 3-phenyI-cyclobutanones7 (60-70% yield) similar to the regiochemistry of ketene cycloadditions, reaction with a series of acrylates and a,/J-unsaturated ketones gave cyclobutanones 5 with regiochemistry opposite to what would be expected from electrostatic considerations of ketene cycloadditions.s... 

The reaction of methylenecyclopropanes with transition metal complexes is well known to promote a catalytic a-ir cycloaddition reaction with unsaturated compounds, in which a trimethylenemethane complex might exist71-76. Recently, much interest has been focused on the interaction of strained silicon-carbon bonds with transition metal complexes. In particular, the reaction of siliranes with acetylene in the presence of transition metal catalysts was extensively investigated by Seyferth s and Ishikawa s groups77-79. In the course of our studies on alkylidenesilirane, we found that palladium catalyzed reaction of Z-79 and E-79 with unsaturated compounds displayed ring expansion reaction modes that depend on the (Z) and (E) regiochemistry of 79 as well as the... 

From the foregoing survey of heterocyclic hydrazonoyl halides, it appears that the main emphasis has been restricted to both their preparation and use as intermediates for further synthesis. Large areas of their chemistry, particularly regarding their physical and biological properties, remain to be developed. A deeper understanding of some aspects of their 1,3-dipolar cycloaddition reactions, such as regiochemistry and site selectivity in terms of the frontier molecular orbital method, is also needed. 

The first photochemical [3 + 2] cycloadditions of arenes with aikenes were reported by two groups in 1966 [76, 77]. This is one of three types of cycloaddition reaction ortho, meta, and para) of arenes with aikenes, and is referred to as the meta-cydoaddition reaction. There are many combinations of arenes and aikenes and, indeed, a large number of reports and reviews have been made on the regiochemistry and stereochemistry [78]. Because this reaction can be used easily to construct a tricyclo[3.3.0.02,8]oct-3-ene framework, the organic syntheses of many natural compounds using this method have been attempted [79], in which the photochemical [3 + 2] cydoaddition plays a key role. 

The Lewis acid ATPH 97 was effective both in rate enhancement and in control of the regiochemistry i n the cycloaddition reactions b etween C,A-diphenyl nitrone (94) and catalytic amount (10 mol%) of 97, the regiochemistry of the 1,3-DC of 94 and 92 was reversed and the isoxazolidine-4-carbaldehyde 95 was obtained as the major cycloadduct in high yield <02TL657>. 

Regiochemistry and stereochemistry When both components of a cycloaddition reaction are unsymmetrically substituted, two regioisomeric cycloadducts are possible. In the case of Diels-Alder reactions, these are shown in the reactions of both C-1 and C-2 substituted dienes and monosubstituted dienophiles. Isomeric adducts can be referred as ortho, meta and para in reference to similar disubstitution isomers of benzene. 

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