Intramolecular pericyclic reactions

Dyotropic rearrangements are uncatalyzed, intramolecular pericyclic reactions in which two (T-bonds simultaneously migrate [158b]. 

Viola, A., Collins, J. J., and Filipp. N Intramolecular pericyclic reactions of acetylenic compounds. Tetrahedron., 37, 3765, 1981. 

Intramolecular pericyclic reactions of acetylene compounds in synthesis of heterocycles 81T3765. 

The triester (38) was needed for an investigation into intramolecular pericyclic reactions between electron-rich (a) and electron-poor (b) double bonds.A Wittig disconnection on double bond (b) (nearer the centre of the molecule) demands an a-dicarbonyl compound whichever way we write it. The keto diester (39) can easily be made from a malonate ester by a-oxidation, so this route is preferred. Further disconnection of phosphonium salt (40) suggests allylic alcohol (41) as intermediate and hence regioselective reduction (Chapter 14) of a,/3-unsaturated aldehyde (42) (Chapter 19). 

An intramolecular pericyclization reaction provides either di- or trisub-stituted pyridines in good yields (Scheme 14) (13EJ02537). Arynes were generated from 2-(trimethylsilyl)phenyltriflate and combined with isocyanides and 3-bromopropyne in one-pot. CsF was the best source of fluoride to provide the benzyne, but yields of the pyridine were low imtil CSCO3 was added. The rate of benzyne formation was modulated with a cosolvent of MeCN and toluene and the ratio of precursors was tweaked. With the optimized conditions, the reaction was shown to be fairly robust in the face... 

A. Viola, J. J. Collins, and N. Filipp, Intramolecular Pericyclic Reactions of Acetylenic Compounds , Tetrahedron, 1981, 37, 3765. 

A recent study of the photoreaction of the thiolactones (67) in the presence of N-phenylmaleimide afforded good chemical evidence for the intermediacy of the ortho-quinoid thioketone (68). Thus the intermediate (68) reacted with the trapping agent only when the former was unsubstituted in the other cases, intramolecular pericyclic reactions occurred prior to intermolecular trapping (Scheme 1) Ano er ortho-quinoid thioketone... 

A complex sequence of pericyclic reactions, intramolecular and intermolecular cycloadditions and cycloreversions, was studied in an attempt to readily achieve bicyclic five-membered heterocycles, the methyl 4,6-dihydrothieno- and methyl-... 

This chapter has taken the reader through a number of microwave-assisted methodologies to prepare and further functionalize 2-pyridone containing heterocycles. A survey of inter-, intramolecular-, and pericyclic reactions together with electrophilic, nucleophilic and transition metal mediated methodologies has been exemplified. Still, a number of methods remain to be advanced into microwave-assisted organic synthesis and we hope that the smorgasbord of reactions presented in this chapter will inspire to more successful research in this area. 

The ene reaction has proved to be particularly powerful in synthesis when carried out intramolecularly. The usual increase in rate for an intramolecular reaction allows relatively unreactive partners to combine. Thus the diene 6.13 gives largely (14 1) the cis disubstituted cyclopentane 6.15 by way of a transition structure 6.14. It is important to recognize that the selective formation of the ci j-disubstituted cyclopentane has nothing to do with the rules for pericyclic reactions. It is a consequence of the lower energy when the trimethylene chain spans the two double bonds in such a way as to leave the hydrogen atoms on the same side of the folded bicyclic structure. This... 

In a Lewis-acid catalyzed intramolecular ene reaction,4 the alkene (ene) reacts in a pericyclic fashion with an enophile to form two new a bonds with concurrent migration of the allylic double bond. 

On treatment with weak base, 1,3-oxathiolane sulfoxide (80) was shown to undergo a reversible intramolecular ene reaction to sulfenic acid (81). Using stronger base, it was found that this pericyclic reaction cannot compete with elimination to enone (82), which then oxidizes or dimerizes to the observed products (83) and (84). The diastere-omeric sulfoxide (79) was found to undergo retro-ene elimination then dimerization to (85) or to (83) and (84) with even a weak base.85... 

Combination of 26 with pericyclic reaction led to a diastereocontrolled synthesis of anthelmintic (-)-kainic acid and anticholinergic (-)-physostigmine and (-)-physovenine. Thermolysis of 68, obtained from (+)-26 furnished diastereoselectively 69 by intramolecular ene reaction. (-)-Kainic acid was obtained from 69 through a sequence of several steps of reactions (Scheme 19).27... 

Frequently, the intramolecular [2 + 2] photocydoaddition of an alkene to a benzene ring is followed by further pericyclic reactions. Such transformations yield... 

An intramolecular reaction of an isocyanate on a benzo[l, 2-b 4,3-6 ]difuran constructed a fused pyridine ring, probably via a pericyclic reaction <86JHC1637>. Photolysis of the bisthiadiazoloquinone... 

Pericyclic reactions involving thiophenes have been utilized to prepare a variety of complex heterocycles. The intramolecular Diels-Alder reaction of 2-vinylbenzo[i]thiophene 92 produced a pair of tetracyclic adducts 93 and 94 <02TL3963>. Coupling of Fischer carbene 96 with 3-alkynylthiophene 95 led to the formation of thieno[2,3-c]pyran-3-one 97 in one step <02JOC4177>. An intramolecular cycloaddition of 97 then afforded tetracyclic adduct 98. A ruthenium-catalyzed cyclodimerization reaction involving bis-thienyl acetylene derivatives was... 

Of course, this was only a theory—until in 1982 K.C. Nicolaou s group synthesized the proposed endiandric acid precursor polyene—and in one step made both endiandric acids D and E, plus endiandric acid A, which arises from a further pericyclic reaction, an intramolecular Diels-Alder cycloaddition of the acyclic diene on to the cyclohexadiene as dienophile. 

One of the very rare examples of a combination of a radical with a pericyclic reaction - in this case a [4+2] Diels-Alder cycloaddition - is depicted in Scheme 3.83 [133]. The seqrtence, elaborated by Malacria and coworkers, is based on the premise that the vinyl radical 3-341 formed from the substrate 3-340 using tributyltin hydride exists mainly in the Z -form. This is reduced by a hydrogen atom to form a 1,3-diene, which can undergo an intramolecular Diels-Alder reaction via an exotransition state reaction (the chain lies away from diene). 

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