3.4- dimethylene-, Diels-Alder reactions

Tamariz and coworkers [42] have described a versatile, efficient methodology for preparing N-substituted-4,5-dimethylene-2-oxazolidinones 42 (Figure 2.5) from a-diketones and isocyanates and have also studied their reactivity in Diels-Alder reactions. This is a method for synthesizing polycyclic heterocyclic compounds. Some of the reactions of diene 42 are summarized in Scheme 2.18. The nitrogen atom seems to control the regiochemistry of the reaction. 

At the other extreme of diene reactivity, some dienes are so unreactive that only powerful dienophiles such as PTAD will undergo Diels-Alder reactions with them. Thus, 4,5-dimethylene-l,3-dioxolan-2-one fails to react with TCNE or maleic anhydride, but gives the required Diels-Alder adduct (63%) with PTAD.232 The powerful dienophilic properties of PTAD have... 

In the majority of the reactions of 3,4-dimethylene-3,4-dihydrofuran with dienophiles, the first reaction involves a conventional Diels-Alder reaction of the furan ring so that the final product, e.g. (340), may incorporate two molecules of the dienophile (93H(35)57) (Scheme 61a). 

Extrusion of sulfur dioxide from a ring-fused dihydiothiophene derivative provides an exceptionally easy method for the preparation of heterocyclic o-dimethylene compounds. These compounds are valuable intermediates in intermolecular Diels-Alder reactions. This extrusion method was used to prepare the o-dimethylene compounds 44-46 <95CC1349> as well as quinolinone derivative 47 <95TL5983>. TTie dihydrothiophene dioxide moiety also played an important role in the formation of the intramolecular Diels-Alder reaction of N-substituted pyrrole 48 <95CC807>. 

We studied the Diels-Alder reaction of levoglucosenone (2) with the very reactive diene 2,3-dimethylene-2,3-dihydrofuran (3),10 the fiiran-based orf/io-quinodimethane, which is readily available by the Flash Vacuum Pyrolysis (FVP) of ester 4.16 Diene 3 dimerizes very rapidly16... 

The dioxin system (effectively a 1,2-ethenodioxy structure) is derivable from catechol and the Diels-Alder reaction shown between one such compound namely 2,3-dimethylene-2,3-dihydro-1,4-benzodioxin and methyl vinyl ketone at 70°C for 15 hours a reaction which has enabled a tricyclic compound in the series to be synthesised in 84% yield (ref. 103). 

Tamariz et al. developed an access to 1-methoxycarbazoles 225 via hydrolysis of 3-arylbenzoxazol-2-ones 223 and subsequent palladium(II)-mediated oxidative cyclization of the resulting diarylamines 224 (Scheme 54) [213, 214]. The required benzoxazol-2-ones 223 are obtained by regioselective Lewis acid-catalyzed Diels-Alder reaction of 4,5-dimethylene-3-aryl-l,3-oxazolidm-2-ones with alk-enes followed by aromatization using 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ). 

This sequence has been used for the synthesis of 6-methoxymurrayanine (232) (Scheme 55) [214]. Reaction of butane-2,3-dione (226) with 4-methoxyphenyl isocyanate (227) to 4,5-dimethylene-3-(4-methoxyphenyl)-l,3-oxazolidin-2-one (228) followed by regioselective boron trifluoride-catalyzed Diels-Alder reaction with acrolein afforded the oxazolone 229. Only minor amounts of the undesired regioisomer were formed. Aromatization using 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) afforded the benzoxazol-2-one 230. Saponification of the cyclic carbamate and subsequent O-methylation led to the diarylamine 231. Finally, oxidative cyclization using stoichiometric amounts of palladium(ll) acetate provided 6-methoxymurrayanine (232). 

The preparation of 2,3-dimethylene heteroaromatics (DMHAs), as analogues of orthoquinodimethanes, has attracted a good deal of attention in recent years. Such systems are often quite labile, and one of the most commonly employed synthetic strategies to them has been to build the required heteroaromatic ring fused to a sulfolene, as a direct precursor to the DMHA [79]. Some interesting work has been done on Diels-Alder reactions from DMHAs derived from fused sulfolenes, and these are covered in more detail in Section 6.5. 

The prominence of the Diels-Alder reaction of quino-2,3-dimethanes (also known as ortfto-quinodimelhanes and < rt/j( -xylylenes) has been established for nearly six decades since the seminal work by Cava [1,2], Several excellent reviews are available [3-7]. The logical extension of this chemistry to the synthesis and Diels-Alder reactions of indole-2,3-quinodimethanes (2,3-dimethylene-2,3-dihy-dro-l/7-indoles) has been recognized as an efficient route to indoles, carbazoles, and related fused indoles [8],... 

This finds some support in a comparison of solution viscosities with polymerization time of a few isolated cases of Diels-Alder polymerization reactions. In the polymerization of 2,5-dimethylene-3,4-diphenylcyclo-pentadieneone with N.N -hexamethylene-fo s-maleimide, the reduced viscosity of the polymer increases from 0.97 after 1 hr to 1.20 after six hours (7). It is necessary to assume that the rate controlling step in this reaction is neither the formation of the initial adduct nor the loss of carbon monoxide. The inherent viscosity of the l,6-6is-(cyclopenta-dienyl)hexane-quinone copolymer increases from 0.10 after sixteen hours to 0.12 after twenty four hours reaction time in refluxing benzene (14). 

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