Of diynes

The first examples of a consecutive radical 5 -exo-/dig-5-exo-dig cyclization of 1,5-diynes have been accomplished by the same researchers [43]. These authors were able to show that their cycloisomerization procedure provides access to strained semicyclic, conjugated dienes with a functionalized dioxatriquinane framework which occurs in the aglycones of steroidal cardiac glycosides, such as isogenine (3-96) [44] and C-norcardanolide (3-97) (Scheme 3.24) [45]. 

Scheme 3.25. Dioxatriquinanes by triethylborane-induced domino radical atom transfer/ cyclization of 1,5-diynes.

The cobalt(I)-mediated [2 + 2 + 2] cycloaddition of 1,5-diynes with mono-alkynes provides access to benzocyclobutene derivatives (Scheme 24). Thermal rearrangement of benzocyclobutenes into o-quinodimethane and subsequent Diels-Alder reaction with an alkene moiety allow the formation of a tricyclic compound. 

In this general area quite efficient intramolecular examples have been described as well (Scheme 6). A thorough examination of the intermediates and likely mechanisms associated with this system has been published. In particular, it has been found that the type of metallacycle formed depends on the length of the chain separating the two alkyne units in the diyne. Efficient intramolecular cycloaddition to a bicy-clic metallacycle occurs only from 1,6- and 1,7-diynes. The poor results on attempted cyclization of 1,5-diynes to give highly strained bicyclo[3.2.0]hepta-l,4-dien-3-ones result from resistance towards closure of the strained four-membered ring upon metallacycle formation. ... 

Thermal rearrangements of 1,5-diynes result in the formation of bisallenes, but the products readily cyclize to give bismethylenecyclobutene derivatives 231122,1123. 

The rearrangement of 1,5-diyne 24 proceeds with high stereoselectivity. Racemic precursor 25 at 310 C (20 Torr) yields a 1 1 mixture (at 400-500 °C 80 20) of products 27 and 28 with less than 1 % of 31. Compound 31 is obtained in 99% yield from meso-291124. 

The formation of 1,5-diyne complexes as side-products in some reactions of cations 92 with metal alkyls [41] and the facile, regioselective reduction of a propargylic chloride complex by Zn/HOAc, used in the synthesis of the insect pheromone 5-(Z)-tetradecenyl acetate [221], provided early evidence for the intermediacy of (propargyl)Co2(CO)5 mdicals. In a similar vein, the binuclear molybdenum-complexed propargyl cations react with Na/Hg to produce... 

This synthetic methodology was used to prepare (+)-estrone in 5 steps and 12.5% yield from 2-methylcyclopentenone where the benzocyclobutene was prepared by a cobalt-mediated cyclization of 1,5-diynes and acetylenes. The reaction type was also used in an alkaloid synthesis. 

Prior to these mechanistic findings by Nolan and coworkers, Hashmi and coworkers found very similar mechanistic principles in their studies of cyclization reactions of 1,5-diynes [31]. In these cyclization reactions, two gold atoms are also needed to dual activate the initial substrate (see Scheme 9.13). Further mechanistic studies also revealed the occurrence of a, in this case even isolable, ew-diarated species, which on their own can serve as catalysts for the studied reactions. These species therefore highlight the importance of organogold compounds. 

Indeed, cw-l,2-divinylcyclopropanes give this rearrangement so rapidly that they generally cannot be isolated at room temperature,though exceptions are known. When heated, 1,5-diynes are converted to 3,4-dimethylenecyclobu-tenes. A rate-determining Cope rearrangement is followed by a very rapid electro-cyclic (18-27) reaction. The interconversion of 1,3,5-trienes and cyclohexadienes... 

It must also be pointed out that the alkyne synthesis through the FVP process can also be extended to bis()5-oxo phosphonium ylides) for the preparation of 1.3-diynes compounds [16]. 

Parker, Raphael, and Wilkinson have investigated a synthetic approach to tropinone (124), which they call the acetylenic route (78). Reaction of hexa-1,5-diyne-l,6-dicarboxylate (145) with methylamine yields the pyrrolidine derivative (146), which by catalytic hydrogenation affords the diester 147 (79,50). 

An unexpected production of 2,4,6-triphenyl-l, 3,5-triazine in the electroreduction of 3,4-diphenyI-l,2,5-thiadiazole 1-oxide has been reported . Synthesis of 1,3-diyne derivatives of 2,4-diamino-l,3,5-triazine, 9a and 9b, has been accomplished by reaction of biguanidine with mono- and di-esters 8a and 8b, respectively <00T1233>. 

Fig. 8 The changes in the NBO re-bond order of in-plane (re , circles) and out-of-plane (rec, diamonds) acetylenic bonds along the IRC pathway for the Bergman cyclization of (Z)-hex-3-ene-1,5-diyne.

Et3B-induced radical cascade reactions with 1,5-enynes and 1,5-diynes have been applied to the synthesis of dioxatriquinanes and tricyclic glucocon-jugates (Scheme 17) [44,45]. Some of these elegant cascade cyclizations were also performed under mild conditions at - 50 °C. 

Dienes are less reactive toward transition metals than enynes and diynes, and perhaps for this reason, the development of effective catalyst systems for the cyclization/hydrosilylation of dienes lagged behind development of the corresponding procedures for enynes and diynes. The transition metal-catalyzed cyclization/hydrosilylation of dienes was first demonstrated by Tanaka and co-workers in 1994. Reaction of 1,5-hexadiene with phenyl-silane catalyzed by the highly electrophilic neodymium metallocene complex Cp 2NdCH(SiMe2)3 (1 mol%) in benzene at room temperature for 3 h led to 5- ///76 -cyclization and isolation of (cyclopentylmethyl)phenylsilane in 84% yield (Equation (15)). In comparison, neodymium-catalyzed reaction of 1,6-heptadiene with phenylsilane led to 5- X(9-cyclization to form (2-methylcyclopentylmethyl)phenylsilane in 54% yield as an 85 15 mixture of trans. cis isomers (Equation (16)). 

Rhodium-catalyzed cyclization/silylation/carbonylation of 1,6-diynes was proposed to occur via a-migratory insertion of CO into the Rh-C bond of dialkylidene cyclopentane intermediate Ik to form acylrhodium alkene intermediate Ilk (Scheme S-endo-[5-M gX2itory insertion of the silylated alkylidene moiety into the Rh-G bond of Ilk would form... 

Aaron L. Odom of Michigan State University has described (Org. Lett. 2004, 6, 2957) a new approach to dialkyl pyrroles. Ti-catalyzed hydroamination of a 1,4-diyne such as 1 leads smoothly to 2. Similarly, Ti-catalyzed hydroamination of a 1,5-diyne such as 3 delivers 4. An inherent limitation of this approach is that it only allows substitution at the 2 and the 5 positions of the pyrrole. 

The Rh4(CO)i2-catalyzed reactions of 1,6-diynes at 95 °C and 20 atm of CO in benzene or acetonitrile are less selective than those catalyzed by Rh2Co2(CO)i2 or Rh(acac)(CO)2, and a different type of silylcarbobicyclization product, azabicyclo[3.3.0]octadienone (351), is formed in addition to 349-type product (352) when benzyldipropargylamines (350) are used (equation 142)339. The formation of a small amount (5%) of the bicyclic pyrrole of 351-type with 7V-tosyl group is also observed besides 348-type (18%) and 349-type (51%) products in the reaction of IV-tosyldipropargylamine339. 

Likewise, no products of the anti-Michael addition were found in the reaction above. Compound 27 was obtained in 28% yield in the case of 1,5-diphenylpenta-l, 4-diyn-3-one (82JOC1968) when bis(ferf-butyldimeth-ylsilyl) telluride was used as the source of telluride anion. Heterocycle 22 (R1 = R2 = Ph) is also formed (in 19% yield). 

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