Correspondence cycloaddition

There has been some investigation of auxiliary-controlled cycloadditions of azir-ines. Thus, camphor-derived azirine esters undergo cycloaddition with dienes, with poor diastereoselectivity [70]. The same azirines were also observed to react unselectively with phenylmagnesium bromide. Better selectivities were obtained when Lewis acids were used in the corresponding cycloaddition reactions of 8-phe-nylmenthyl esters of azirine 2-carboxylates (Scheme 4.48) [71]. The same report also describes the use of asymmetric Lewis acids in similar cycloadditions, but mediocre ees were observed. 

Fig. 31 Energy diagram showing the relationship between AAv, the change in the charge transfer complex absorption energy for a series of substituted anthracene-TCNE complexes, and the change in activation barrier, AE, for the corresponding cycloaddition reactions...

Summary The formation of reactive intermediates via dehalogenation of chlorosilanes was investigated by using lithium powder and sonication. Whereas in the absence of a diene substrate mainly polysilanes are obtained, reactions with, e.g., dimethylbutadiene, yield the corresponding cycloaddition products, indicating silylenes and silaethenes as intermediates. 

Benzocyclopropene (88) and naphtho[h]cyclopropene react with various alkenic substrates to give cy-cloaddidon piquets in the presence of silver ion. Reaction of (88 equation 39) with butadiene assisted by 1 mol % of AgBp4 in benzene (0 C, 30 min) gives (89) and (90). Dimethylallene (91 equation 40) and oct-4-yne (95 equation 41) also react readily with (88) in the presence of silver ion to give the corresponding cycloaddition products (92), (93) and (96), together with acyclic products (94) and (97). 

There have only been a few new reports of reactivity of this type. The Paterno-Buchi reaction of 92 with substituted pyruvates gives the endo- and < ti-products shown (Equation 2) <1996TL1195>. In a later study, the corresponding cycloadditions of l,3-dioxol-2-one with a variety of aldehydes and ketones were also examined <2007ARK(viii)58>. 

When dienophile I (R2 = COOCH3) and diene 3 (R1 = CH3) are reacted under these conditions, the corresponding cycloaddition product 4 (R = CH3 R2 = COOCH3) is obtained in 94% yield and with 94% ee. 

All these processes are retrodimerizations. The characteristic of the corresponding cycloadditions is that the sensitizer, while being essential for activating the reactants (by photoinduced electron transfer) and for making the products persist (by back electron transfer), takes no part in the cycloaddition proper, the key step of which is the combination of a radical ion with its parent molecule. For cycloadditions of unlike species, another mechanistic pathway is open, namely, via direct combination of radical ion pairs, as was reported in several recent publications [117],... 

Planar chiral ortho substituted benzaldehyde chromium complexes are useful compounds for a variety of asymmetric reactions. For example, planar chiral tricarbonylchromium complexes of o-substituted benzaldehydes were reacted with Danishefsky s diene in the presence of Lewis acid at room temperature to afford the chromium-complexed 2,3-dihydro-4-pyranones 25 with high dias-tereoselectivity (Eq. 15) [14]. The high diastereoselectivity of the formation of cycloaddition products 25 is also contributed to an exo-side approach of the diene to anti-oriented carbonyl oxygen of the planar chiral ortho substituted benzaldehyde chromium complexes. When the reaction takes place at lower temperature, aldol-type condensation product 26 was obtained along with the formation of pyranones. The open intermediate 26 was easily transformed to the corresponding cycloaddition product after stirring at room temperature [14]. 

As previously noted, the typical high temperatures and long reaction times required for the normal electron demand Diels-Alder reactions with electron-poor indoles have rendered such approaches less attractive for the synthesis of more sensitive and highly functionalized substrates in total synthesis. Recently, there have been a few reports on attempts to accelerate these cycloaddition processes. For example, Piettre and coworkers investigated the activation of the dienophilic indoles under high pressure [33]. Thermal Diels-Alder reactions of lV-tosyl-indole-3-car-boxaldehyde (65) with dienes 66a and 66b (195°C, sealed tube, 72 h) resulted in conversions of 67% and 25%, respectively (Scheme 18). By increasing the pressure for the above reactions to 16 kbar (48 h, 50°C), the corresponding cycloadditions with dienes 66a and 66b resulted in conversions of 93% and 86%, respectively. 

If an ester group was introduced into the terminal position of the alkyne illustrated below, a rapid intramolecular Diels-Alder reaction took place smoothly in the absence of any catalyst and the corresponding cycloaddition adducts were obtained <01TL3171>. 

The reaction can be applied to other olefinic substrates such as isopropenyl acetate and butyl vinyl ether. The reactions with cyclohex-2-enone give the corresponding cycloaddition products in 64 and 47% yields, respectively (Scheme 5.7). 

Cycloadditions of the Parent Dienes 2a and 2p. The results are summarized in Table I. First, it must be noted the considerable rate enhancements of the aqueous cycloadditions using the unprotected dienes 2 (3.5 hrs at room temperature with methacrolein) were observed when compared with the corresponding cycloadditions conducted in toluene with the peracetylated dienes 8 (168 hrs at 80°C with the same dienophile). Second, a virtually complete endo stereoselectivity was obtained in water... 

The cycloaddition of ketones with nitrile ylides proceeds much more slowly than the corresponding cycloaddition with aldehydes. On the other hand, ketones with electron-withdrawing groups such as trifluoromethyl, ethoxycar-bonyl, nitrile, or phosphonate at the a position react rapidly with the 1,3 dipole. All of this is understandable in terms of frontier MO theory. Nitrile... 

The same catalytic effect imparted by the empty capsule was observed changing the diazoacetate ester in the reaction with acrolein. In fact, with the t-butyl derivative the reaction with the capsule led to 97% yield, and with benzyl diazoacetate 54% yield of the corresponding cycloaddition product was observed. In both cases, the presence of the capsule occupied by the ammonium inhibitor led to a marked decrease of product formation. The higher conversions obtained in the presence of t-butyl diazoacetate as substrate were probably due to the good affinity with the cavity by means of CH-ji interactions. 

Inda-Box-Cu(OTQ2] (56). The corresponding cycloaddition products, allyl dihydropyrancarboxylates (57), were obtained with good diastereoselectivities (12 1) and 86-97% ee. 

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