Vinylic cations, cyclization

The reverse reaction (that is, the oxidation of a vinyl radical by Fe to the corresponding vinyl cation) may be involved in the reaction of the dimethyl ester of acetylenedicarboxyUc acid 261 with Fenton s reagent [Fe —H2O2, (217)] (216). When 261 was treated with Fe —H2O2 and the reaction mixture was extracted with ether, a small amount of furan 262 was isolated. A possible mechanism (216) for its formation may be addition of hydroxyl radical to the triple bond of 261, followed by addition of the intermediate vinyl radical to a second molecule of 261 and oxidation of the resulting radical with Fe to the corresponding vinyl cation, followed by cyclization to 262, as shown in Scheme XX. 

Support for the involvement of Fe comes from the observation that the kinetic chain length of the dimethyl ester increases with increasing concentration of added Fe together with a fourfold increase in the yield of 262 (216). However, as a vinyl cation bearing carbonyl substituents may be energetically unfavorable, an alternative mechanism, involving a ligand transfer from hydrated Fe ions followed by an acid-catalyzed cyclization, may be a more likely pathway ... 

In both oxa- and aza-alkyne Prins cyclization an unexpected halide exchange with halogenated solvents presumably caused by the vinyl cation intermediates was observed [37]. From a synthetic point of view, it is important to use the correct combination of FeXs and X-containing solvent in order to avoid the undesired halide scrambling (Scheme 28). 

When a cyclization sequence is terminated by an alkyne, vinyl cations are formed. Capture of water leads to formation of a ketone.15... 

Barluenga et al. developed a novel exo-endo-cyclization of a,codiynes by use of bis(pyridyl) iodonium(I)-tetrafluoroborate (IPy2BF4) as catalyst.1171 The reaction follows a cationic-cationic pathway (scheme 6). First the electrophilic iodo ion reacts with the triple bond of 26 to give the relative stable vinyl cation 28. Ring closure leads to a seven-... 

Conjugated ketones and esters react with allenylsilanes to yield acylcyclopentenes (Eq. 9.60) [63]. These products are formed by initial 1,4-addition to the conjugated double bond to afford a silyl-stabilized vinyl cation intermediate. 1,2-Silyl migration gives rise to a second silyl-stabilized vinyl cation which cyclizes to the acyl cyclopen-tene (Scheme 9.14). 

Volume 75 concludes with six procedures for the preparation of valuable building blocks. The first, 6,7-DIHYDROCYCLOPENTA-l,3-DIOXIN-5(4H)-ONE, serves as an effective /3-keto vinyl cation equivalent when subjected to reductive and alkylative 1,3-carbonyl transpositions. 3-CYCLOPENTENE-l-CARBOXYLIC ACID, the second procedure in this series, is prepared via the reaction of dimethyl malonate and cis-l,4-dichloro-2-butene, followed by hydrolysis and decarboxylation. The use of tetrahaloarenes as diaryne equivalents for the potential construction of molecular belts, collars, and strips is demonstrated with the preparation of anti- and syn-l,4,5,8-TETRAHYDROANTHRACENE 1,4 5,8-DIEPOXIDES. Also of potential interest to the organic materials community is 8,8-DICYANOHEPTAFULVENE, prepared by the condensation of cycloheptatrienylium tetrafluoroborate with bromomalononitrile. The preparation of 2-PHENYL-l-PYRROLINE, an important heterocycle for the synthesis of a variety of alkaloids and pyrroloisoquinoline antidepressants, illustrates the utility of the inexpensive N-vinylpyrrolidin-2-one as an effective 3-aminopropyl carbanion equivalent. The final preparation in Volume 75, cis-4a(S), 8a(R)-PERHYDRO-6(2H)-ISOQUINOLINONES, il lustrates the conversion of quinine via oxidative degradation to meroquinene esters that are subsequently cyclized to N-acylated cis-perhydroisoquinolones and as such represent attractive building blocks now readily available in the pool of chiral substrates. 

Vinyl acetate- Acetic acid vinyl ester Acetic acid ethenyl ester, (108-05-4), 65, 135 VINYL CATION EQUIVALENT, 66, 95 Vinyl cation synthons, 66, 102 Vinyl radical cyclization, 66, 81, 82 VINYLAT10N OF EN0LATES. 66, 95, 104 Vinyl lithium, 66, 53, 55... 

Alkynes are readily acylated with acid chlorides under Friedel-Crafts conditions to form, in most cases, fram-p-chlorovinyl ketones through the corresponding vinyl cation intermediate [Eq. (8.16)]. The first study in 1935 reported low yields.11 Later in acylations with acyl triflates, p-keto vinyl triflates were obtained in satisfactory yields.123 When aroyl derivatives are used, the intermediate can undergo cyclization to form indenones. Chlorovinyl ketones formed from terminal alkynes may also react further losing hydrogen chloride to yield conjugated acetylenic ketones 11,13... 

A cationic cyclization inspired by a biogenetic postulate (Scheme 44) (73TL3963) results in formation of equal amounts of the C-15 epimers (189) from the vinyl epoxide (188). 

A previously proposed mechanism for the acid-catalysed cyclization of 5-cyclodecynone (268) to 272 that involved hydration of the intermediate vinyl cation (269 — 270) has now been ruled out, since no significant incorporation of 180 could be observed when the reaction was carried out in H2180. Instead, a new mechanism has been proposed, involving the cyclization of 269 to 271 followed by fragmentation of the C—O bond396. 

Cleavage-cationic cyclization of bromocyclopropanes. Bromocyclopropanes sub-st i tuted by a suitably situated hydroxyl or carboxyl group undergo cleavage-cyclization at 25° in the presence of 1-1.5 equiv. of AgOCOCF3 or Hg(OAc)2. Vinyl lactones, tetrahydropyranes, or tetrahydrofuranes can be prepared in this way.1... 

A new route to ( )-trachelanthamidine (6) and ( )-isoretronecanol (5) has been reported by Nossin and Speckamp.12 Cyclization of the ethoxy-lactam (28), which is formed on reduction of the imide (27), led to ring-closure with the acetylene to give, after hydrolysis, a 4 1 mixture of the epimers (29) and (30) (Scheme 7). The preferential formation of the five-membered ring is considered to be due to stabilization of an exocyclic vinyl cationic intermediate by the phenylthio group. Reduction of the separated diastereoisomers (29) and (30) yielded ( )-trachelanthamidine (6) and ( )-isoretronecanol (5), respectively. 

A stereochemical study of the synthesis of unsaturated 1,4-aminoalcohols via the reaction of unsaturated 1,4-alkoxyalcohols with chorosulfonyl isocyanate revealed a competition between an retentive mechanism and an SnI racemization mechanism, with the latter having a greater proportion with systems where the carbocation intermediate is more stable.254 An interrupted Nazarov reaction was observed, in which a nonconjugated alkene held near the dienone nucleus undergoes intramolecular trapping of the Nazarov cyclopentenyl cation intermediate.255 Cholesterol couples to 6-chloropurine under the conditions of the Mitsunobu reaction the stereochemistry and structural diversity of the products indicate that a homoallylic carbocation derived from cholesterol is the key intermediate.256 l-Siloxy-l,5-diynes undergo a Brpnsted acid-promoted 5-endo-dig cyclization with a ketenium ion and a vinyl cation proposed as intermediates.257... 

It is proposed that cyclization of the methyl trienyne proceeds via formation of the preferred linear vinylic cation A rather than the less stable bent vinylic cation B (Figure Si5.6). Introduction of the trimethylsilyl group, however, overrides this preference by stabilizing the silicon (3 carbocation C relative to the a carbocation D. 

Annelation azulenesA new route to azulenes involves reaction of tropylium cations with allenylsilanes, which generates a vinyl cation (a). This cation cyclizes to a cycloheptadienyl cation, which can be converted to a dihydroazulene by elimination of H+. However, the presence of (excess) tropylium cation favors aromatization to an azulene. Protodesilylation of the allenylsilane can be effected by use of trimethoxymethylsilane as an acid scavenger. Yields are low ( 25%)... 

The first step is the formation of a symmetrical allyl cation, which then initiates the cyclization. The next double bond is disubstituted so that it has no built-in regioselectivity but prefers to form a six-member ed rather than a five-member ed ring B. The next double bond is trisubstituted and directs the formation of a six-membered ring C. The alkyne, being linear, can reach only through its inner end and so a five-membered ring D is formed. The resulting linear vinyl cation picks up a molecule of water to... 

It is proposed that the electrophilic iodo ion first reacts with one triple bond in 1-126 to give the relative stable vinyl cation 1-128. Ring closure leads to a seven-mem-bered ring 1-129 containing another vinyl cation moiety, which then cyclizes to produce the final tricycle 1-127. 

This cyclization has obvious applications to the synthesis of steroids and indeed Johnson et al. applied this reaction to a synthesis of dZ-progcsterone. The key step in the synthesis involves the cyclization of (3) to give (4). This reaction was carried out with trifluoroacetic acid as above, but ethylene carbonate was added to the reaction to trap the vinyl cation. After cyclization potassium carbonate was added to hydrolyze the enol complex. In this way (3) was converted into (4) in 71 % yield. The tetracyclic ketone (4) was converted into progesterone (6) by ozonization followed by intramolecular aldol condensation. Nole that (4) is a 5 1 mixture of the 17/7- and 17a-epimeric ketones. The mixture was converted into (6) and then separated by fractional crystallization. 

Cyclization occurs during the reaction of l,2-bis(phenylethynyl)bcnzene (190) with a wide variety of reagents for example, reaction with bromine furnishes 192. Presumably, the intermediate vinyl cation 191 is attacked by Br from the least... 

Vinyl ether radical-cation cyclizations using appended electron-rich aryl rings as the nucleophilic component have also been explored [149]. The nature of the products obtained was found in part to be a function of the position of electron donors on the... 

Clearly the course of these cyclization reactions is dependent upon the silicon group. In this regard, cyclization of (82) affords the steroid nucleus (83 Scheme 40). The formation of (83) was attributed, in part, to a transition state preference for the formation of the linear vinyl carbocation (84b) rather than the bent vinyl cation (84a), which would be produced in an endocyclic cyclization. The formation of (81) was controlled by the generation of the -silyl carbocation (85a), which may be a precursor to an a-silyl ketone, which undergoes protodesilylation. It is not known whether the formation of (81) as the major cyclization product occurs through a kinetic pathway or by Wagner-Meerwein rearrangement of the kinetically prefened linear carbocation (85b). 

In many respects the Pummeter reaction can be regarded as the sulfur version of the Polonovski reaction (and vice versa), and by analogy to the Polonovski reaction the central intermediate is a sulfur-stabilized carbocation (thionium ion). Although the existence of this species is only transient, it reacts to give a number of different products, e.g. a-acetoxy sulfides, vinyl sulfides, cationic cyclization products, etc., depending upon the sulfoxide structure and reaction conditions. Other reaction pathways ate specific to the Pummerer reaction as a result of sulfur s ability to expand its valence shell (additive Pum-merer reactions). A moderate degree of asymmetric induction is also observed in certain Pummerer reactions, where optically pure sulfoxides are substrates. 

---