Cycloaddition yields

The ring-opening of the cyclopropane nitrosourea 233 with silver trifiate followed by stereospecific [4 + 2] cycloaddition yields 234 [129]. (Scheme 93) Oxovanadium(V) compounds, VO(OR)X2, are revealed to be Lewis acids with one-electron oxidation capability. These properties permit versatile oxidative transformations of carbonyl and organosilicon compounds as exemplified by ring-opening oxygenation of cyclic ketones [130], dehydrogenative aroma-tization of 2-eyclohexen-l-ones [131], allylic oxidation of oc,/ -unsaturated carbonyl compounds [132], decarboxylative oxidation of a-amino acids [133], oxidative desilylation of silyl enol ethers [134], allylic silanes, and benzylic silanes [135]. 

The reaction kinetics between a maleimide derivative and a 3,5-hexadiene derivative varies depending on the maleimide compound being reacted. Cycloaddition yields of greater than 80 percent and often as much as 90-95 percent can be expected within 1-18 hours at room temperature or slightly elevated reaction conditions (e.g., 30°C). 

Dipolar addition is closely related to the Diels-Alder reaction, but allows the formation of five-membered adducts, including cyclopentane derivatives. Like Diels-Alder reactions, 1,3-dipolar cycloaddition involves [4+2] concerted reaction of a 1,3-dipolar species (the An component and a dipolar In component). Very often, condensation of chiral acrylates with nitrile oxides or nitrones gives only modest diastereoselectivity.82 1,3-Dipolar cycloaddition between nitrones and alkenes is most useful and convenient for the preparation of iso-xazolidine derivatives, which can then be readily converted to 1,3-amino alcohol equivalents under mild conditions.83 The low selectivity of the 1,3-dipolar reaction can be overcome to some extent by introducing a chiral auxiliary to the substrate. As shown in Scheme 5-51, the reaction of 169 with acryloyl chloride connects the chiral sultam to the acrylic acid substrate, and subsequent cycloaddition yields product 170 with a diastereoselectivity of 90 10.84... 

Detailed mechanistic information concerning an intramolecular arylalkene cycloaddition yielding cyclobutane derivatives via a radical cation process gener-... 

Chinese chemists have reported the synthesis of pentacyclo[4.3.0.0 , 0 ]nonane-2,4-bis(trinitroethyl ester) (88). This compound may find potential use as an energetic plastisizer in futuristic explosive and propellant formulations. The synthesis of (88) uses widely available hydroquinone (81) as a starting material. Thus, bromination of (81), followed by oxidation, Diels-Alder cycloaddition with cyclopentadiene, and photochemical [2 - - 2] cycloaddition, yields the dione (85) as a mixture of diastereoisomers, (85a) and (85b). Favorskii rearrangement of this mixture yields the dicarboxylic acid as a mixture of isomers, (86a) and (86b), which on further reaction with thionyl chloride, followed by treating the resulting acid chlorides with 2,2,2-trinitroethanol, gives the energetic plastisizer (88) as a mixture of isomers, (88a) and (88b). Improvements in the synthesis of nitroform, and hence 2,2,2-trinitroethanol, makes the future application of this product attractive. 

Mechanisms that are probably associated, respectively, with these processes are (i) the formation of betaine intermediates (306) (Fig. 3) 103,143,149,197,200 homolysis or heterolysis of the X—Z bond (304) or the X—Z bond (305) giving diradicsd (307) or dipolar (308) intermediates, (iv) 1,3-dipolar cycloaddition yielding intermediate adducts (e.g, 309), The base-catalyzed rearrangements (ii) present very interesting mechanistic problems suitable for speculation and experimental enquiry. 

The intramolecular cycloadditions of cychc nitronates have received much more attention. The cyclic nitronate structure provides three basic modes of intramolecular cycloaddition (Fig. 2.15). Attachment of the tether to the C(3) position of the nitronate results in the formation of a sprro system (sprro mode). However, if the tether is appended to the C(4) position of the nitronate, the dipolar cycloaddition yields a fused ring system (fused mode). Finally, if the tether is attached at any other point of the cyclic nitronate, the cycloadducts obtained will consist of bicyclic structures (bridged mode). 

By comparison with these less interesting reactions, photocyc-loadditions of alkynes to alkenes and related compounds provide a fascinating extension of the analogous reactions for alkenes. The simplest type of cycloaddition yields a cyclobutene (2.99), but depending on the relative absorption characteristics of the substrates and product at the wavelength of irradiation, the cyclobutene may... 

Boedeker and Courault215 reacted Schiff bases (151) derived from 2-aminopyridine and aromatic aldehydes with diphenylketene. In benzene at room temperature 4-oxopyrido[l,2-a]pyrimidines (152) were obtained in a reversible [4 + 2] cycloaddition reaction, whereas upon boiling mesitylene, irreversible [2 + 2] cycloaddition yielded azetidinones (150). Previously Sakamoto et al.216 prepared the azetidinones (150) in boiling xylene. 

Scheme 3.114). The use of another wavelength (A < 313 nm) in the case of the second reaction leads to a similar [2 + 2] cycloaddition yielding the corresponding polycyclic compounds 346 [371]. 

The reaction of diphenylketene with 1,3,2-diazaboroles 234 in hexane at — 20 °C leads to the formation of 1,3,2-oxazaborolidines 238 in 64-70% yield. The reaction proceeds via the coordination of the ketene oxygen to boron 236 followed by a [2+3] cycloaddition yielding the bicyclic intermediate 237. Fission of B-N bond leads to the product formation as yellow to colorless crystals (Scheme 40) <20000M5791>. 

Numerous photodimerization studies of 1,3-cyclohexadiene 36 have been reported (Sch. 9). Thermal cycloaddition yields a 4 1 mixture of endo/ exo [4+2] adducts 37 and 38 in modest yield. Irradiation of the diene in cyclohexane near its 2max of 254 nm yields very little dimer, but irradiation at 313 nm leads to a mixture of dimers, favoring the [2+2] adducts 39 [37]. The use of y-radiation produces similar mixtures [38,39]. A triplet sensitizer leads to largely the [2+2] adducts plus exo 38 and little of the endo [4+2] isomer 37 [40]. When the photochemistry is conducted in the presence of the electron acceptors anthracene 41, LiC104-42 or pyrylium 43, only [4+2]... 

Some hetero double bond systems have been shown to enter [3 + 2] cycloaddition reactions with the mesoionic 1,3-dithiolones. Thus, the mesoionic 1,3-dithiolones (2) react with formaldehyde, prepared in situ by depolymerization of paraformaldehyde, with regiospecific formation of the 2-oxa-6,7-dithiabicyclo[2.2.1]heptanone derivatives (131). The corresponding reaction of (2) with the N=N double bond of dimethyl azodicarboxylate proceeds via cycloaddition yielding (132), and a similar reaction takes place between (2) and 4-phenyl-l,2,4-triazoline-3,5-dione (78CB3171). 

Aza Diels-Alder reactions ofW-acyl-l-aza-1,3-butadienes have also been studied and applied in synthesis [236-239]. Interestingly, Jung et al. demonstrated the possibility to generate N-acyl- 1-aza-l,3-butadienes like 3-27 by flash vacuum pyrolysis of 2-azetines such as 3-26. Subsequent cycloaddition yielded the bicyclic product 3-28 which is a useful intermediate in alkaloid synthesis [240] (Fig. 3-9). 

In the case of cyclic tertiary enaminones as dienophiles in the reaction with nitroalk-enes as heterodienes, [4 + 2]-cycloaddition yields tetrahydrobenzo >][l,2]oxazin-8-ones nearly quantitatively. Heating in aprotic solvents affords substituted diaster-eoisomeric pentalenones by ring contraction of the six-membered ring326 (equation 244). 

The other 1,2,3-triphosphabuta-l,3-diene is yet not known as a stable compound. It has been suggested to be an intermediate during reactions given in Eq. (44a,b), but an intramolecular [2 + 2] cycloaddition yielded the first 1,2,3-cyclobutane (96, 97). 

In addition to providing excellent diastereoselection in the cyclocondensation process, the chiral oxa-zolidinone auxiliary can be reductively removed to give the enantiomerically pure 3-amino-3 lactam in good overall yields (Scheme 16). Homochiral 1,4-imidazolidinones also give cis 3-lactams with excellent induction when the asymmetric carbon atom is next to the nitrogen atom (15 in Scheme 17). High inductions can also be observed with a -y-lactam (16), but the cycloaddition yields a mixture of cis and trans isomers. The level of diastereoselection is much lower with imide synthons (17). 

Formation of aminocyclopropanes in a carbene cycloaddition to a carbon-carbon double bond containing no nitrogen function requires an amino-substituted carbene as counterpart. Such a type of reaction was observed with special nitrile ylides possessing a suitable double bond within the molecule Thus, nitrile ylides (193) generated from the precursors 191 or 192, underwent intramolecular [2-f-l] or [2-I-3]cycloaddition yielding 194 and 195 respectively (Scheme 4). 195 was easily hydrolyzed to aminocyclo-propane (196). The preferential direction of the cycloaddition was influenced by the nature... 

Irradiation of the dione (343) affords the two products (344. SOX) and (345, 15X). The formation of these is presumed to follow the path outlined in Scheme 11 where monodecarbonylation affords the keto diene (346) which undergoes an intramolecular Diels-Alder addition affording the minor product (344). A second decarbonylation of (346) yields the tetraene (347) which photocyclizes by a (4+4)-cycloaddition yielding the polycyclic compound (345). - ... 

In the first step, formyl ketone 14a reacts via its enol form 14b with tosyl azide in a 1,3-dipolar cycloaddition yielding intermediate 15. Its triazoline ring resolves into the a-diazo ketone 4 and 7V-formyl tosyl amide. 

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