Quinone monoimide

Engler and coworkers [76] developed a new domino process which consists of a [5+3] cycloaddition of a p-quinone monoimide with a styrene derivative followed by a [3+2] or [3+3] cycloaddition. The reaction allows the formation of two additional rings and up to eight stereogenic centers, with high selectivity. The best results, with 58% yield of4-230, were obtained in the transformation of 4-227 and 4-228 in the presence of BF3 Et20 at -20 °C (Scheme 4.49). In addition, the diastereomer 4-231 was obtained in 16 % yield. It can be assumed that the cation 4-229 functions as an intermediate. The process also functions with quinones, though much less efficiently. 

The actual synthesis (Scheme 10) commenced with the couphng of diene acid 54 and aminophenol 53 to provide diene amide 55. In situ generation of quinone monoimide 52 under oxidative conditions and subsequent intramolecular Diels-Alder (IMDA) reaction furnished an 8 1 mixture of endo/exo... 

In the absence of a Lewis acid promoter, the 2 + 3-reaction between p-quinone monoimide and azadienes, e.g. a,-unsaturated hydrazones yielded 2,3-dihydroben-zofurans in moderate to excellent yields when the dienophile was maintained at low concentration.43... 

Oxidative demethylation A new approach to indoloquinones such as 5 involves Michael addition of ethyl acetoacetate to the quinone monoimide 1 to give 2, which is dehydrated to the indole 3 in 73% overall yield. The latent quinone ring is then modified to give the p-methoxyaniline 4. The final step involves the oxidative demethylation reaction of Rapoport (4, 431-432) to give an intermediate quinone imine, which is hydrolyzed to 5. 

Engler has noticed an interesting dichotomy in his indole synthesis involving benzoquinone monoimides <97TL6135>. For instance, while reaction of the quinone monoimide 96 with N-phenylsulfonyl enol ethers in the presence of Ti(IV) Lewis acid catalysts affords the expected tetrahydrocarbolines 97, reactions promoted by BF3 produce substituted benzofurans. 

Acylindoles act as dienophilic components of a [4 + 2] cycloaddition when treated with an o-quinone monoimide (Equation (85)) <87TL669l>. 

JOC6155>. Thiophene itself reacts in an inverse electron demand Diels-Alder with o-quinone monoimide to give products derived from intermediate (100) <93H(35)1I25>. 

Likewise, the oxidative dearomatization ofpara-methoxy substituted N-protected anilines 288 using (diace-toxyiodo)benzene in the presence of methanol gives p-quinone monoimide ketals 289 (Scheme 3.120) [358]. If the oxidation of aniline derivatives is performed in the presence of water, the final isolated products are the respective / -benzoquinones or p-benzoquinone monoketals resulting from the hydrolysis of initially formed monoimide ketals 289 [358,359]. 

A direct access to the synthesis of dihydroquinolines (eq 22) was developed. The vinyl quinone mono- or di-SES-imide, accessible by oxidation of the corresponding aminophenol or phenylenediamine, undergoes a thermal 67r-electrocyclization in the presence of a polar nonprotic additive. Dehydrodesulfinylation of the SES group will lead to the quinoline. The vinyl quinone monoimide substrate can also provide the protected indole by photochemical cyclization. 

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