Conjugated diynes synthesis

A recent synthesis of a conjugated triendiyne, fraws-bupleurynol, exemplifies the use of the conjugated diyne synthesis developed above to the synthesis of natural products121,153 (Scheme 55). 

SCHEME 53. Synthesis of conjugated diynes via Pd-catalyzed alkynylation of 1,2-dihaloethylenes96... 

SCHEME 54. Pd-catalyzed strictly pair-selective two-step synthesis of unsymmetrically substituted conjugated diynes... 

A similar coupling of alkynyliodonium tosylates 132 with dialkynylcuprates 134 leads to conjugated diynes 135 (Scheme 55) [114,115]. This method can be used for the preparation of unsymmetrical diynes in moderate yield. Recently, this coupling was employed in the synthesis of various liquid-crystalline diaryl-diacetylenes [115]. 

Dithiins are also synthesized from conjugated diynes using benzyl thiol reductive debenzylation of intermediate 153 by sodium in liquid ammonia at 70 C gives, after aerial oxidation, the 1,2-dithiin 154 (Scheme 77) <1967AGE698>. Additional examples of 1,2-dithiins synthesis utilizing this approach are provided in CHEC-III . 

Further extension of the method has led to a one-pot procedure for the synthesis of symmetrical and unsymmetrical conjugated diynes in 60-70% yields, as shown in Scheme 12. The bulky 1,2-dimethyl propyl group has proved to possess a low... 

The synthesis of conjugated diynes via the Glaser coupling reaction " is the classical method for homocoupling of terminal alkynes. The coupling reaction is catalyzed by CuCl or Cu(OAc)2 in the presence of an oxidant and ammonium chloride or pyridine to yield symmetrically substituted diynes. " The oxidative dimerization appears to proceed via removal of the acetylenic proton, formation of an alkynyl radical, and its dimerization. 

A similar behaviour was observed in the reaction of lithium dialkynylcuprates with alkynylaryl iodonium tosylates leading to conjugated diynes. Unsynunetrical diynes (98) were also prepared by this method. However, the selectivity was moderate, due to the competitive formation of the symmetrical diynes (99).202 This reaction has been recently applied to the synthesis of various liquid-... 

Much use has been made of conjugated diynes, which are also at the oxidation level of 1,4-dicarbonyl-compounds, which react smoothly with hydrosulhde or sulhde, under mild conditions, to give 3,4-unsubstituted thiophenes. Unsymmetrical 2,5-disubstituted thiophenes can be produced in this way too. ° Since nearly all naturally occurring thiophenes are found in plant genera, and co-occur with polyynes, this laboratory ring synthesis may be mechanistically related to their biosynthesis. 

Conjugated diynes (cf. 5, 346). A convenient route to these substances, both symmetrical and unsymmetrical, has been developed by Sinclair and Brown. Treatment of 1-alkynyldisiamylboranes with lithium alkynes forms an ate complex (I), which on reaction with iodine at - 78" in THF gives a diync (2). It is also possible to carry out this synthesis without isolation of the l-alkynyl-... 

Conjugated diynes, R C=C—C=CR . Pelter et al. have extended the synthesis of symmetrical conjugated diynes (6, 294) to a synthesis of unsym-metrical conjugated diynes (equation I) in which this reagent is used as the carrier borane. 

Acetylenic chemistry has continued to have an important role in pheromone synthesis. A synthesis of (z, z )-11, 13-hexadecadiena1, the major component of the sex pheromone of the female navel orangeworm, Amyelois tvansitella, x i Qd reduction of an unsymmetri cal conjugated diyne with dicyclohexylborane with >99.5% selectivity 132... 

Schulte [285, 286] and later Chalk [287] described the Cu(I)-catalyzed synthesis of symmetrical 2,5-diarylpyrroles 261 from conjugated diynes 257 and primary amines 258. The reaction is believed to proceed via the transition metal-catalyzed hydroamination [33, 288-291] leading to tautomeric aminoenyne 259 or homopro-pargylic imine 260 intermediates, which further undergo 5-endo-dig cyclization to furnish pyrrole product 261 (Scheme 8.95). 

Scheme 3.40 Highly selective synthesis of conjugated diynes by Negishi coupling [168].

Synthesis of Polyynes. Conjugated diyne and polyyne units, which exhibit unusual electrochemical, optical, and structural properties, can be efficiently constructed from 1-haloalkynes via homologation by one acetylene unit with TIPS-acetylene through a transition metal-catalyzed cross-coupling reaction, 2 which allows an access to a new class of polyyne framework. 

Sinclair, J.A., and H.C. Brown Synthesis of Unsymmetrical Conjugated Diynes via the Reaction of Lithium Dialkynyldialkylborates with Iodine. J. Org. Chem. 41,1078 (1976). 

This reaction is also used in the synthesis of carboranylbenzenes. The 2-carboranylenynes 2.233 react with conjugated diynes 2.234, forming polysubstituted carboranylbenzenes 2.235 with a good yield (40-71%) (Scheme 2.79) [3]. 

---