Unsaturated cydoadditions reaction

The theoretical studies showed that the [2 4-2] cydoaddition reactions with alkenes or alkynes were almost barrierless and very exothermic. The exothermicity for alkynes (—42.8 kcal mol ) was found to be greater than that for alkenes (—21.3 kcal mol ). Scheme 4.21b shows the calculated [2 4- 2] cydoaddition with polar unsaturated molecules, formaldehyde and FICN. The barriers (3.6kcalmol for formaldehyde and 9.0 kcal mol for HCN) are slightly higher than those in the reactions with alkenes and alkynes. The exothermicity is also significant (—26.5 for formaldehyde and —37.4 for HCN). 

Polymerization by Cycloaddition. Bisimides and oligoimides capped with reactive unsaturations such as maleimide, acetylene, and xylylene groups, can be chain-extended by a cydoaddition reaction with proper bisdienes. 

The 1,3-dipolar cycloaddition reaction between nitrones and unsaturated systems under the action of microwave irradiation has been shown to be a powerful method for the synthesis of a wide variety of novel five-membered heterocycles. In 2002, several nitrone-mediated 1,3-dipolar cydoaddition reactions were reported by de la Hoz in the original edition of this book [3jj. Most of the examples provided had been described between 1995 and 2001 [9a, 13b, 84, 92]. 

Azide additions to a,P-unsatnrated systems are another method for the preparation of 1,2,3-triazoles. Cydoaddition of aryl azides to a,P-unsaturated aldehydes 88 in the presence of catalytic diethylamine and DBU afforded 1,4-disubstituted-l,2,3-triazoles 89 via an inverse electron-demand process (13CC10187). Michael addition of sodium azide with ethyhdene bisphospho-nates 90 in cydoaddition reactions via sonication afforded bisphosphono-1,2,3-triazoles 91 (13T4047).A one-pot protocol for the synthesis of 1,2,3-triazoles was prepared from unactivated alkenes with azidosulfenylation of the carbon-carbon double bond followed by the copper-catalyzed azide—alkyne cycloaddition (13JOC5031). 1,5-Disubstituted-l,2,3-triazoles 93 were synthesized from enamides 92 with tosyl azide (13AG(E)13265). Reaction of ethyl 3-(alkylamino)-4,4,4,-trifluoro-but-2-enoates 94 with mesyl azide in the presence of DBU afforded l,2,3-triazole-4-carboxylates 95 (13EJ02891). 

Basically, the metathesis reaction consists of reversible [2 + 2]-cydoaddition processes between two unsaturated partners, leading to equilibrium mixtures where kinetic, thermodynamic, and entropic parameters are decisive. The main steps are summarized in Scheme 2.1. 

Another example is the conversion of cyclohexene (164) to the bicyclic sultone (165) (Scheme 67). This allows the synthesis of sultones with partially determined stereochemistry. Both reactions depend on the high leaving group properties of the methanesulfoxy anion. Unsaturated 8-sultones like (166) can be obtained by 1,4-cydoaddition of sulfenes (generated in situ) to a,(3-unsaturated carbonyl compounds (Scheme 68). 

Oppolzer and coworkers [147, 454] have developed a class of reagents based on the enantiomeric bomane-2,10-sultam skeleton 1.133. These chiral auxiliaries are easily prepared from the enantiomeric 10-camphosulfonic adds [455]. Saturated or a,P-unsaturated TV-acylsultams 1.134, occasionally prepared from Af-silyl precursors [396], have been used very frequently. Asymmetric alkylations, animations and aldol reactions of enolates or enoxysilane derivatives of 1.134 (R = R CH2) [147, 404, 407, 456-460] are highly selective. The a,(3-unsaturated TV-acylsultams 1.134 (R = R R"C=CH) suffer highly stereoselective organocuprate 1,4-additions [147, 173], cyclopropanations [461], [4+2] and [3+2] cydoadditions [73,276,454,462], OSO4 promoted dihydroxylations [454,463] and radical addi-... 

Among other carboxylic acid derivatives, the a,p-unsaturated lactams 1.92 proposed by Meyers and cowotkers [327] give useful results. No catalysis is needed in cydoadditions with silyloxydienes reactions with 2,3-dimethylbutadiene (R = R I Me) (F %... 

Cydoadditions are particularly useful reactions and are the most effective methods for generating cyclic and heterocydic compounds from unsaturated precursors by forming two bonds in a single operation [1], As such, they are extremely useful for construction of many natural or biologically active molecules. 

The Bose research group has developed a large variety of [2+2] cycloaddition reactions of ketenes with imines to form yS-lactams [16, 112, 113]. In 1991, Bose et al. described the synthesis of a-vinyl yS-lactams by [2-p2] cydoaddition of a,p-unsaturated acyl chlorides with a SchifF base in chlorobenzene. Reaction times were reduced from several hours with conventional methods to 5 min under the action of microwaves [112], and yields of the desired adducts were improved from very low to approximately 70% when compared with classical methods. 

Furthermore, 12 was found to catalyze [3+2] cycloaddition of allenes with various -substituted a,P-unsaturated ketones to produce enantiomerically enriched, highly functionalized cydopentenes possessing two contiguous stereocenters [41], Along with this line, Marinetti reported that 2-aryl-l,l-dicyanoethylenes were also good reaction partners of the allenoates in the 12-catalyzed cydoaddition [42] (Scheme 7.24). 

Intramolecular [2+2]-photocycloadditions of cycUc a,P-unsaturated enones with remote double bonds have been extensively used to synthesize a variety of interesting compounds, including natural products. An analysis of the mechanism of the additions has also been carried out. 2-Pyrones having pendant enes and dienes undergo synthetically useful photocycloaddition processes to give tricyclic lactones and lactone-bridged cyclooctadienes by [2+2]-and [4+4]-cydoadditions, respectively. The photochemical reactions... 

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