2-ethynyl-5- furan

The isomerization of vinyl- or ethynyl-oxiranes provides a frequently exploited source of dihydrofurans or furans, but analogous conversions of vinylaziridines have not been applied so often. While most of the examples in Scheme 87 entail cleavage of the carbon-heteroatom bond of the original heterocycle, the last two cases exemplify a growing number of such rearrangements in which initial carbon-carbon bond cleavage occurs. 

The 17a-ethynyl compound (59) has been prepared in 88% yield from estr-4-ene-3,17-dione (58) and acetylene, at 2-3 atm pressure in tetrahydro-furan in the presence of potassium t-butoxide. Presumably the A-ring enone system is protected as the enolate anion during the course of the reaction. 

Furan has served as a starting material for the synthesis of methyl rt-tetradeca-frans-2,4,5-trienoate (177), a compound presumed to be a sex attractant of the male dried bean beetle (72TL3777). Conversion of furan to 4,4-dimethoxy-frans-but-2-en-l-al (174) was accomplished in two steps by a previously reported procedure. Intermediate (174) was then ethynylated, acetylated and reacted with lithium dioctylcuprate to yield allene (176). Hydrolysis of the acetal and Mn02-NaCN oxidation of the aldehyde afforded the methyl ester (177 Scheme 37). 

BTMSBD reacts with a variety of nucleophiles to give novel heterocycles such as selenophen tellurophen and pyrazoles. It has also been used 1n [2+4] cycloaddition/cycloreversion sequences to prepare ethynyl-substituted Pyridazlnes and furans. ... 

The isomerization of ethynyl-oxiranes 223 leads to furans 224, as illustrated in Scheme 128 <1969JCC12>. A similar reaction of ethynyl-oxiranes with amines gives pyrroles (see Scheme 86 in Section 4.2.3.3.1). Aziridine 225 reacts with bis(trimethylsilyl)acetylene upon heating to provide pyrrole 226 (Scheme 129) <1999JOC1630>. 

As illustrated in the following scheme, trifluoromethyl-substituted furans were generated in high yields by cyclization of ethynyl trifluoromethyl allyl alcohol in the presence of AgOTf as catalyst <07EJO3916>. The starting material can be obtained easily from 1,2-unsaturated trifluoromethyl ketones. 

DIENOPHILES Acrolein. Ethynyl p-tolyl sulfone. 3-Methylsulfonyl-2,5-dihydro-furane. 3-Nitro-2-cyclohexenone. Phenyl vinyl sulfone. 

The presence of two trifluoromethyl groups in the molecule increases the acidity of a-proton of 3,4-bis(trifluoromethyl)furans. In the reaction of 2-ethynyl-3,4-bis(tri-fluoromethyl)furan 216 with ketones 217 both propyn-1 -ols 218 and triarylmethanols 219 were obtained and converted into the corresponding salts by treatment with perchloric acid."... 

Furan-2-yl)-pyrroles were readily ethynylated with acylbromoacetylenes on solid AI2O3 medium (no solvent, room temperature, 1 h) to afford 5-(furan-2-yl)-2-acylethynylpyrroles in 39-74% yields. In the case of 2-(furan-2-yl)pyr-roles, an alternative ethynylation of the furan ring took place, in which the ratio of the furan and pyrrole ring ethynylation products was 1 5-7 (14T9506). 

Rhodium(lll)-catalyzed redox—neutral coupling of N-phenoxyacet-amides and alkynes led to benzo[l)]furan derivatives (13AGE6033). Furo[2,3-l)]pyran-6-one derivatives were prepared via rhodium(II)-catalyzed reactions of diazo compounds and ethynyl compounds (13T9294). Copper-mediated oxidative annulation of phenols and unactivated internal alkynes afforded benzo[l ]furan derivatives (13CS3706). E t-kaurane maoecrystalV was produced via C-H functionalization (13JA14552). Rhodium-catalyzed intramolecular C-H... 

A soln. of 6-acetoxy-2-ethynyl-2,5,7,8-tetramethylchroman in abs. tetrahydro-furan added drop wise at 15-20° under N2 during 30-40 min. to a soln. of ethyl-magnesium bromide in the same solvent, methylene chloride added to avoid resin formation, refluxed 1 hr., cooled to room temp., treated portionwise with Dry Ice followed by isolation of the product which includes treatment with NH4GI and HGl 2-carboxyethynyl-6-hydroxy-2,5,7,8-tetramethylchroman. Y 74.5-94%. H. Mayer et al., Helv. 46, 650 (1963). 

Lithiation of ethynyl(trifluoromethyl)furans 275 followed by the reaction with diaryl ketones produced diaryl(2-arylethynyl)carbinols 276 and triarylcarbinols 277. Further treatment with HCIO4 yielded the corresponding dyes 278, 279. Similarly, furan 280 was transformed into dye 281 [159]. 

Ethynylated 2/f-cyclohepta[()]furan-2-ones 486 are prepared by Pd-catalyzed alkynylation of 3-iodo-5-isopropyl-2//-cyclohepta[ ]furan-2-one 487 with the corresponding ethynylarenes [292]. The compound reacts with tetracyanoethy-lene (TCNE) 488 and TCNQ 25 to afford highly polarized dicyanoethylene derivatives 489 and 490 respectively (Scheme 6.128) [293]. 

A divergent behavior has been shown by o-arylethynyl-phenols [109] and o-ethynyl-phenols [110] under their Pd-catalyzed carbonylative reactions with vinyl triflates. For example, treatment of o-(phenylethynyl)phenol 64 and 3,3,5,5-cyclohex-l-en-l-yl triflate 65 under usual carbonylative conditions (Scheme 9.31) gives the 3-acylbenzo[h]furan 66 along with the ester 67. 

---