Acetylenes 2 + 2 -cycloadditions involving

Acetylene dicarboxylate and maleic anhydride failed to react with simple methylene cyclopropenes, but reacted readily with calicene derivatives, as shown by Prinz-bach293. Thus ADD combined with benzocalicene 497 to give the dimethyl tri-phenylene dicarboxylate 499, whose formation can be rationalized via (2 + 2) cycloaddition across the semicyclic double bond as well as (4 + 2) cycloaddition involving the three-membered ring (498/501). The asymmetric substitution of 499 excludes cycloaddition of ADD to the C /C2 triafulvene bond (500), which would demand a symmetrical substituent distribution in the final triphenylene derivative. 

In another conceptually novel [5 + 2]-process, Tanino and co-workers synthesized cycloheptene derivatives by stereoselective [5 + 2]-cycloadditions involving hexacarbonyldicobalt-acetylene complexes as the five-carbon component and enol ethers as the two-carbon component (Schemes 22 and 23).60 61 The role of the dicobalthexacarbonyl complex is to facilitate formation and reaction of the propargyl cation putatively involved as an intermediate in this reaction. The dicobalthexacarbonyl moiety can be removed using various conditions (Scheme 24) to provide alkane 60, alkene 62, and anhydride 63. 

All other cases of the pyran syntheses so far described involve one molecule of an ynamine as the acetylenic reactant. Simple [An + 27t] cycloaddition involving an ynamine and an a,j8-unsaturated aldehyde or ketone constitutes a general approach to N,N-disubstituted 6-amino-4//-pyrans 93 the stability and hence the yields depend on substituents R to R5 (Table... 

A major initial limitation of the benzocyclobutene approach to o-quinodimethanes was the lack of efficient, large-scale syntheses for many specifically substituted derivatives. Fortunately, recent developments have lemov much of this impediment. Q>nceptually, the synthesis of benzocyclobutenes from aromatic precursors can be envisaged in only a limited number of ways. These include [2 -i- 2] cycloadditions involving benzynes and alkenes, intramolecular cyclization on to a benzyne, cyclizations involving arene anions, and electrocyclic closure of o-quinodimethanes. Benzocyclobutene derivatives can also be prepared by aromatization of bicyclo[4.2.0]octanes. Detailed discussion of variations to these approaches can be found in the cited reviews. The cobalt catalyzed co-oligomerization of 1,5-hexadiynes with al-kynes, especially bis(trimethylsilyl)acetylene, has also been employed for the preparation of specifically substituted benzocyclobutenes. In the latter case the cyclobutenes are often not isolated but converted directly to o-quinodimethanes and subsequent products. ... 

Hexafluoroacetone azine reacts with two equivalents of terminal olefins (71JCS(C)2404) and with acetylene derivatives (75TL1125), forming 1,5-diazabicy-clo[3.3.0]octenes and l,5-diazabicyclo[3.3.0]octa-2,6-dienes 27, respectively (Scheme 27). The cycloaddition involves two [3 + 2] processes. The structure of azamethineimine is confirmed by X-ray analysis data (74AGE475). 

Stabilization of the oxaphosphete formed by the cycloaddition of the P = O group and the acetylene moiety involves formally the rupture of the P-O bond and the formation of a P = C and a C = O double-bond (Fig. 16). It is recalled that a Wittig reaction follows the opposite direction the cycloaddition of a P = C and O = C unit gives a 1,2-oxaphosphetane that is opened up to furnish a phosphine oxide and an olefin (Fig. 16). 

Cycloaddition involving ketene intermediates continue to provide a useful entry to cyclobutanones, and Snider et al. have given fuller details of the potential for Lewis acid-catalysed thermal [2 + 2]-cycloadditions involving acetylenes in the synthesis of cyclobutenes. 

One of the most general and useful reactions of alkenes and acetylenes for synthetic purposes is the addition of electrophilic reagents. This chapter is restricted to reactions which proceed through polar intermediates or transition states. Several other classes of addition reactions of alkenes are of importance and these are discussed elsewhere. Nucleophilic additions to electrophilic alkenes were covered in Chapter 1 and cycloadditions involving concerted mechanisms will be encountered in Chapter 7. Many of the free-radical addition reactions of alkenes are considered in Chapter 12 of Part A. 

Indoles are usually constructed from aromatic nitrogen compounds by formation of the pyrrole ring as has been the case for all of the synthetic methods discussed in the preceding chapters. Recently, methods for construction of the carbocyclic ring from pyrrole derivatives have received more attention. Scheme 8.1 illustrates some of the potential disconnections. In paths a and b, the syntheses involve construction of a mono-substituted pyrrole with a substituent at C2 or C3 which is capable of cyclization, usually by electrophilic substitution. Paths c and d involve Diels-Alder reactions of 2- or 3-vinyl-pyrroles. While such reactions lead to tetrahydro or dihydroindoles (the latter from acetylenic dienophiles) the adducts can be readily aromatized. Path e represents a category Iley cyclization based on 2 -I- 4 cycloadditions of pyrrole-2,3-quinodimcthane intermediates. 

Doubt (75ZN(B)822) has been cast on a number of claims for the formation of 2-azetin-4-ones from cycloaddition of activated isocyanates to acetylenes (70TL119). The simple 2-azetin-4-one (246) was not isolated or even detected directly at -50 °C in the photofragmentation of compound (245), but indirect evidence for its formation was the isolation of adducts (248 X = MeO, MeNH) in the presence of methanol or methylamine (75TL1335). The most convincing evidence for an isolable 2-azetin-4-one involves treatment of the... 

The addition of 1,3-dipoles to alka-l,3-diynes has been studied in less detail than that to conjugated alkadienes and alkenynes (80UK1801). Conjugated diynes get involved in [2- -3]cycloaddition at the unsubstituted acetylene bond. 

Substituted 1-benzoxepins can be obtained by the cycloaddition of activated acetylenes to the benzofuran system. When 2-(Af-mcthylanilino)benzofurans is treated with dimethyl acetylene-dicarboxylate, substituted 1-benzoxepins 1 are obtained in reasonable yield.180,181 This reaction presumably involves a 2a,7b-dihydrocyclobuta[6]bcnzofuran as an intermediate (see Section 4.2).182... 

The Diels-Alder reaction of 2-vinylfurans 73 with suitable dienophiles has been used to prepare tetrahydrobenzofurans [73, 74] by an extra-annular addition these are useful precursors of substituted benzofurans (Scheme 2.29). In practice, the cycloadditions with acetylenic dienophiles give fully aromatic benzofurans directly, because the intermediate cycloadducts autoxidize during the reaction or in the isolation procedure. In the case of a reaction with nitro-substituted vinylbenzofuran, the formation of the aromatic products involves the loss of HNO2. 

The acetylenic amide 294 is converted on irradiation in benzene into two oxidized dimers 295.243 A complex pathway involving a [ 4 + Kl cycloaddition followed by oxidation and photorearrangement has been proposed. 

During the course of a study of the cycloaddition of azidomethyldiethylphospho-nate with acetylenes and enamines leading to alkyltriazoles under solvent-free conditions we observed that specific effects can be involved, depending on the nature of the substituents on the dipolarophiles [50] (Eq. (7) and Tab. 3.4). 

The reactions of alkyl nitronates (164) or (165) derived from a-functionalized primary AN with monosubstituted acetylenes produce mixtures of diastereomeric aziridines (166) in moderate to high yields. Most probably, the first step of this process involves normal concerted cycloaddition to give the corresponding intermediates A, which were not detected due to their fast rearrangement to give acyl-substituted aziridines (166). The reaction is regioselective and stereospecific. The latter fact was demonstrated by French researchers (95). 

Thermolysis of 44 produced products derived from the Myers-Saito cyclization reaction. However, when 43 having a trimethylsilyl substituent at the acetylenic terminus was subjected to heating in the presence of 1,4-CHD at 70 °C for 3 h, the 1H-cyclobut[a]indene 46 was produced. A reaction mechanism involving an initial Schmittel cyclization to generate the benzofulvene biradical 45 followed by an intramolecular radical-radical coupling was proposed to account for the formation of the formal [2 + 2]-cycloaddition product 46. 

The cycloaddition reactions of 2-phenyloxazol-4(5H)-one with acetylenic dipolarophiles has been briefly reported. " The formation of 2-phenylfurans may well involve a tautomerism analogous to that exhibited by azlactones (76 77). 

Schindler and coworkers verified the formation of hydroxyl radicals kinetically and further RRKM calculations by Cremer and coworkers placed the overall concept on a more quantitative basis by verifying the measured amount of OH radical. An extensive series of calculations on substituted alkenes placed this overall decomposition mechanism and the involvement of carbonyl oxides in the ozonolysis of alkenes on a firm theoretical basis. The prodnction of OH radicals in solution phase was also snggested on the basis of a series of DFT calculations . Interestingly, both experiment and theory support a concerted [4 4- 2] cycloaddition for the ozone-acetylene reaction rather than a nonconcerted reaction involving biradical intermediates . 

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