Furan Diels-Alder adducts

Benzo[Z)]furans and indoles do not take part in Diels-Alder reactions but 2-vinyl-benzo[Z)]furan and 2- and 3-vinylindoles give adducts involving the exocyclic double bond. In contrast, the benzo[c]-fused heterocycles function as highly reactive dienes in [4 + 2] cycloaddition reactions. Thus benzo[c]furan, isoindole (benzo[c]pyrrole) and benzo[c]thiophene all yield Diels-Alder adducts (137) with maleic anhydride. Adducts of this type are used to characterize these unstable molecules and in a similar way benzo[c]selenophene, which polymerizes on attempted isolation, was characterized by formation of an adduct with tetracyanoethylene (76JA867). 

The 1,2 naphthalyne can be trapped by reaction with excess furan to give the Diels-Alder adduct In the absence of furan, 1,2-naphthalyne can react with heptafluoro-2-naphthyllithium to yield a mixture of monohydrotndecafluoro-binaphthyls (equation 14)... 

Subsbtuting vinylic hydrogen in a,P-unsaturated carbonyl compounds with vinylic fluonne does not affect their dienophilic character negatively Indeed, 3,3-difluoroacrylic acid is more reactive toward furan than its nonfluonnated counterpart [95] (equation 81) Consistent with this observation is the fact that tctrafluorobenzoquinone forms only a bis-Diels-Alder adduct m 68% yield in its reaction with cyclopentadiene at room temperature [96, 97 ... 

The heterocyclic aryne (trithiadiazepyne) 12.18, generated by treatment of the monobromo derivative of 12.16 with a strong base, can be trapped as a Diels-Alder adduct with furan (Scheme 12.3). ... 

Liquid-phase photolysis of furan atroom temperature occurred in very low yields (1 % conversion), giving a mixture of Diels-Alder adducts deriving from the reaction of cyclopropene-3-carbaldehyde and formylallene with furan (85JOC3034). 

The heteroaryne 1 is trapped as the Diels-Alder adduct when 6-bromo-l,3/4, 5,2,4-trilhia-diazepine is treated with various dienes in the presence of N./V-diisopropylethylamine or sodium methoxide, e.g. trapping of 1 with furan to give 2.391... 

Stable cA-1-phenyl-1-cyclohexene 24 photodimeiizes via Diels Alder cycloaddition to trans adduct 25 (Equation 1.33) [66] and the photoexcitation of dihydrobenzofuran-fused cyclohexenone 26 in net furan gives the trans fused Diels-Alder adduct 27 (Equation 1.34) [67]. 

From the Diels-Alder Adduct of Furan and Acrylic Acid. 29... 

The first three carba-sugars were synthesized by McCasIand and coworkers. Two other carba-sugars were prepared from myoinositol, and the remaining eleven carba-sugars have been synthesized from the Diels-Alder adduct of furan and acrylic acid. Conformational assignments of the carba-sugars were established with the aid of H-n.m.r. spectroscopy. 

All sixteen of the racemic carba-sugars predicted are known, as well as fifteen of the enantiomers. The most accessible starting-material for the synthesis of racemic carba-sugars is the Diels-Alder adduct of furan and acrylic acid, namely, e i o7-oxabicyclo[2.2.1]hept-5-ene-2-carboxylicacid (29). Furthermore, adduct 29 is readily resolved into the antipodes, (—)-29 and (+)-29, by use of optically active a-methylbenzylamine as the resolution agent. The antipodes were used for the synthesis of enantiomeric carba-sugars by reactions analogous to those adopted in the preparation of the racemates. 

The Diels-Alder adduct, anhydride 63, on treatment with hydrazine hydrate in acetic acid for 20 min yielded 252 which on keeping in acetic acid for further 8 h gave the imide derivative 253. This adduct, imide 253, underwent retro-Diels-Alder reaction to eliminate furan giving amino maleimide derivative 254 (Scheme 45) <2005J OC4553>. 

Another strained alkylidenecyclopropane, 2-methylbicyclo[3.1.0]hex-l-ene (21), formed by cyclization of the carbenoid generated from dibromide 20 at 0 °C gave, in the presence of an excess of 1,3-diphenylisobenzofuran (10), a very small amount (5%) of a 2 1 mixture of diastereoisomeric Diels-Alder adducts 22 (Scheme 4) [11a]. The parent furan does not capture 21 even when used as solvent for the carbenoid cyclization. 

Furans add to DEAZD to give Diels Alder adducts, although short reaction times are essential if the initial adduct is to be isolated. Attempts to convert the adduct into the bicyclic compound 116 failed. The only product isolated was a trimer of 4,5-dihydropyridazine, possibly formed as shown in Scheme 16.178 However, the furan adducts are readily converted... 

In another approach to periplanone B by Cauwberghs and De Clercq, an intramolecular Diels-Alder reaction of furan-allene 122 afforded a mixture of two exo adducts 123 and 124 and an endo adduct (not pictured) in 90% yield and a 5 4 1 ratio (Scheme 19.23) [28], Refluxing the mixture in mesitylene (N2, 164 °C) afforded a 2 1 equilibrium mixture of 123-124 through a cydoreversion process. The Diels-Alder adduct 123 was converted to 125 via a series of synthetic manipulations, which constituted a formal total synthesis of periplanone B (126). 

Furan reacted with a -bromoacrolein in the presence of 10 mol% of catalyst 364 to give the Diels-Alder adduct 373 in 98% yield with 92% ee (equation 109)235. Cycloadduct 373 has been applied in further synthesis236. The related catalyst (S)-369b proved much less effective in this reaction. 

Nair and coworkers have described the [8 + 2] cycloaddition reactions of 2H-cyclohep-ta[fr]furan-2-ones such as 521 in several reports311. The reactions of 521 with alkenes yield azulene derivatives upon extrusion of carbon dioxide. Table 30 summarizes the results of the reactions between 521 and some 6,6-disubstituted fulvenes 522 (equation 151)311b. In the case of 6,6-dialkyl fulvenes 522a-c, the [8 + 2] cycloadducts 523 were the major adducts obtained, the Diels-Alder adducts 524 only being formed in trace amounts. 

Enzymic discriminations of enantiopic groups of weso-compounds have been widely exploited in asymmetric synthesis. Ohno and co-workers reported a total synthesis of L- and D-ribose, showdomycin and cordycepin based on the optically active half-esters 56 and 57, which were obtained enzymatically (Pig Liver Esterase PLE) from the corresponding mcso-diesters 54 and 55, respectively, derived from the Diels-Alder adduct 53 of furan to dimethyl acetylenedicar-boxylate. The diesters 58 - 60 were also found to be good substrates for PLE leading to the corresponding half-esters 61 - 63 with high enantiomeric purities. ... 

In their pioneering work. Just and co-workers have described many interesting transformations of the Diels-Alder adducts of furan to methyl nitroacrylate (77 + 77 ) and to dimethyl acetylenedicarboxylate (53). The mixture of racemic adducts 77 + 77 was hydroxylated into the exo-cis-diols 125 + 125 , separable by crystallization. Treatment of the isopropylidene acetal obtained from 125 with diazabicyclo[5.4.0]undec-5-ene (DBU) gave a high yield of alkene 126. Ozonolysis followed by a reductive work-up with dimethylsulfide, then with NaBH4, gave a mixture of epimeric triols 127. Cleavage with sodium periodate afforded 2,5-anhydro-3,4-0-isopropylidene-DL-allose (128) in 15 % yield, based on methyl 2-nitroacrylate used. TTie same allose derivative was obtained from adduct 53. ... 

Another useful synthetic application of a Diels-Alder adduct of the chloro ester 1-Me is the facile preparation of the spirocyclopropanated analog 75 of so-called naked sugar 76 [50] (Scheme 19) [30]. The transformation of the adduct endo-59a of 1-Me onto furan (57) into a-chloroamide 74 followed by fragmentation of the latter under basic conditions allowed to prepare the versatile building block 75 in 71 % overall yield. Starting from isomer exo-59a, the yields were 82 and 52% for the first and the second step, respectively. The analogous trans-... 

Scheme 19. Useful chemical transformations of Diels-Alder adducts endo-59a and 56b, respectively, of the chloro ester 1-Me onto furan (57) and 77-Boc-protected pyrrole (55b) [30]...

Thiete sulfones show an irregular behavior pattern when involved in cycloaddition reactions. With 1,3-dienes, dienamines, enamines, ynamines, diazoalkenes, cyclopropadiene, and its substitution products, furan, and anthracene, the addition proceeds in the normal fashion. With certain Diels-Alder reagents such as tetraphenylcyclopentadienone (tetracycloneX however, the cyclic sulfones react anomalously. The Diels-Alder adducts undergo decomposition with SO 2 and CO extrusion to a seven-membered ring, the tetraphenylcycloheptatriene 223. Bicyclic octadienone is produced as well (Eq. 62). The mechanism of this unusual reaction is proposed by... 

Considerable effort has heen directed towards the synthesis of monosaccharide derivatives, and, in particular, C-glycosyl compounds, from Diels-Alder adducts offuran. Just and his group investigated the reaction of furan with a number of dienophiles, and described many interesting transformations of the adducts obtained from... 

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