Other Syntheses using Acetylenes

Other Syntheses using Acetylenes. a-(Alkylideniminoxy)-ketones (80) are obtained by addition of strong electrophiles to alkynes in nitromethane solution by a mechanism which is presumed to involve a (2,3)-sigmatropic rearrangement of the first formed intermediate (81). Using acyl cations, e.g. (X = MeCO+), an additional product is the / -diketone (82), which is the only product in the case of monosubstituted alkynes.  

The propargylic alcohol (83) has been cleverly isomerized to the ajff-unsaturated aldehyde (84) by equilibrating its tetrahydropyranyl ether with the corresponding allenyl derivative in the presence of strong base. After selective hydrolysis of the allenyl acetal (vinyl ether) (85), (84) is easily separable from unchanged (86) by distillation.  

The diacetylenic amine (87) gives the piperidone (88) by treatment with dimethylamine, or the pyrrolenine (89) by hydration-cyclization.  

Silver carbonate on celite (Fetizon s reagent) quantitatively converts steroidal 17-ethynyl alcohols, e.g. (90), or cyanohydrins, e.g. (91), into the corresponding ketones (92) at the same rate at which allylic hydroxy-groups are oxidized.  

Unsaturated acid chlorides (93) react with monosubstituted alkynes to give mixtures of compounds from which 4-alkylidenecyclopentenones (94) can be separated, which are otherwise not readily accessible.  

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Most commercial enone synthesis methods are not applicable to all desired enones. The acetylene condensation/hydration method can only make enones with even numbers of carbons, while formaldehyde condensation preferentially gives branched products at higher molecular weights. Other methods use expensive oxidation reagents and start with the allylic alcohols. Yet other methods use even more expensive reagents, " or electrochemical methods... 

Thermoplastic polymers were also earlier reported to have been prepared by heating tricyclodecane dithiol with terephthaloyl chloride in a chlorinated solvent [16b]. Additional examples are cited for polythioesters based on bis-phenol A and random other polythioesters derived from bicyclic thiols. The starting diolefins for the dithols are prepared by the Diels-Alder synthesis using cyclopentadiene, acetylene, butadiene, etc. 

The monograph presents a modern methodology of regioselective synthesis of aromatic and nonaromatic carbocyclic and heterocyclic ring systems based mainly on [2+2+2] and [4+2] cycloaddition, and other reactions of acetylenic units, including enediynes and enyne-allenes. The features of the construction of polyaromatic structures, including heterostructures, based on cycloaddition reactions is used in the synthesis of advanced materials including nanomaterials. 

Negishi and coworkers have developed a method for preparation of conjugated -enynes via alkenylboranes. In one synthesis (227), acetylene (50i) was treated with disiamylborane and then with the lithium salt of acetylene (503) to afford complex (504), which after treatment with iodine and sodium hydroxide stereoselectively afforded (505) (Scheme 88). Syntheses of other conjugated E,Z dienes have been achieved by this method (228, 229). By use of thexylborane and 1-bromo-l-alkynes, conjugated E,E dienes can be readily prepared (230) (Scheme 89). 

A small amount of acetylene is used in condensations with carbonyl compounds other than formaldehyde. The principal uses for the resulting acetylenic alcohols are as intermediates in the synthesis of vitamins (qv). 

This excess hydrogen is normally carried forward to be compressed into the synthesis loop, from which it is ultimately purged as fuel. Addition of by-product CO2 where available may be advantageous in that it serves to adjust the reformed gas to a more stoichiometric composition gas for methanol production, which results in a decrease in natural gas consumption (8). Carbon-rich off-gases from other sources, such as acetylene units, can also be used to provide supplemental synthesis gas. Alternatively, the hydrogen-rich purge gas can be an attractive feedstock for ammonia production (9). 

A number of alternative multi-step procedures for the synthesis of a-tert-alkyl ketones are known, none of which possess wide generality. A previous synthesis of 2-tert-penty1cyclopentanone involved reaction of N-1-cyclopentenylpyrrol 1 dine with 3-chloro-3-methy1-l-butyne and reduction of the resulting acetylene (overall yield 46 ). However, all other enamines tested afford much lower yields. Cuprate addition to unsaturated ketones may be useful in certain cases. Other indirect methods have been briefly reviewed. ... 

Other interesting synthetic applications of the ketone-derived enamine alkylation are found in the monomethylation of steroid enamines (249), extension of the benzylation reaction (250) to a ferrocene derivative (251), the use of a-bromoesters (252) and ketones (252) or their vinylogues (25J), in the syntheses of alantolactone (254-256), isoalantolactone (257), and with a bridged bis-enamine (258). The use of bifunctional alkylating agents is also seen in the introduction of an acetylenic substituent in the synthesis of the characteristic fragrant constituent of jasmine (259), the synthesis of macrocyclic ketolactones (260), the use of butyrolactone (261), and the intermolecular or intramolecular double alkylations of enamines with dihalides (262). 

Oxetanes are present in several biologically active natural compounds as, for example, the taxol ring skeleton. An interesting method used to obtain this particular ring is the thermal [2 -i- 2] cycloaddition reaction. Longchar and co-workers reported a novel [2-1-2] cycloaddition of /1-formil enamides 5, often used in other cycloaddition and condensation processes, with acetylenic dienophiles 6 under microwave irradiation (in a domestic oven) to afford ox-etenes 7 in 80% yields [29]. This reaction was directed towards the synthesis of D-ring annelated heterosteroids (Scheme 2). 

The fruit of a shrub that grows in Sierra Leone is very toxic and has been used as a rat poison. The toxic principal has been identified as Z-18-fluoro-9-octadecenoic acid. Suggest a synthesis from 8-fluorooctanol, l-chloro-7-iodoheptane, acetylene, and any other necessary organic or inorganic reagents. 

Very many naturally occurring 3-alkyl- or 3-acylfurans are now routinely synthesized by preparing a substrate for treatment with 3-furyllithium, itself made from 3-bromofuran according to Fukuyama, Tokoroyama, and Kubota, who used it to obtain pyroangensolide and fraxinellone.212 Equally, 2-lithiofuran is used for such natural products as the acetylenic furans from Alphonsea ventricosa213 and other compounds.214 For the synthesis of the sesquiterpenoid sponge-metabolite pleraplysillin-2 78, the lithiofuran 79 and therefore the bromofuran 80 was needed to secure the orientation a suitable preparation was devised for 80.215... 

Using the unsymmetrically substituted acetylene Me3SiC=CPh, the kinetically favored substituted complex 8a is formed initially, cycloreversion of which gives the symmetrically substituted and thermodynamically more stable product 8b. Due to steric reasons, the other conceivable symmetric product 8c is not formed [9]. Such metallacycles are typically very stable compounds and are frequently used in organic synthesis, as shown by the detailed investigations of Negishi and Takahashi [lm], Bis(trimethylsilyl)acetylene complexes are a new and synthetically useful alternative. 

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