Routes from alkynes

Hydroboration-protonolysis is thus a general, stereospecific and versatile route from alkynes to alkenes, enynes and dienes. 

An alternative approach to the formation of p-enamino esters was through conjugate addition of a primary amine to an alkyne (eq. 51).57-59 This complementary method provided a route from alkyne 205 to P-enamino ester 233, which underwent aza-annulation with acryloyl chloride to give 234. Catalytic hydrogenation efficiently gave the cis substituted tetrahydropyridone species (235). 

Representing an impressively straightforward route to bicyclo[5.3.0]decadiene derivatives, Martin and co-workers have designed a one-step route to the [5 + 2]-cycloadducts from alkynes and VCP building blocks through the use of a sequential [RhCKCO k-catalyzed allylic alkylation/[5 + 2]-cycloaddition (Equation (9)). 

In the hopes of accessing similar molecules that would contain C-6 functionalization, our group explored the cycloisomerization of such alkyne hemiketals as 152.70 Compound 152 was prepared by addition of ethynyltrimethylsilane to 5-0-terf-butyl-diphenylsilyl-2,3-0-isopropylidene-D-ribonolactone (151), followed by desilylation (25% over two steps). Trie thy lamine-mediated cycloisomerization71 provided an oxepinone, compound 153, in 41% yield. 1,2-Reduction of the enone functionality followed by acetylation under standard conditions provided 154 in 56% yield over two steps as a 3 1 ratio of diasteromers (the favored diastereomer is shown in Scheme 24). A small group of oxepines were prepared by this method. Variability in the yield of the cyclization step, which was moderate at best, has prevented this route from being applied more generally for the preparation of oxepines. 

RCM of dienes to cycloalkenes provides a useful method for the syntheses of carbo- and heterocycles and thus has been proved to be extremely effective in total synthesis of various natural products. Usually, however, mixtures of (E)- and (Z)-olefms result. In contrast, ring-closing alkyne metathesis provides a reliable route for synthesis of both (E)- and (Z)-macrocycloalkenes in a stereoslective manner taking advantage of stereoselective partial reduction of resulting cycloalkynes. A Lindlar reduction gives (Z)-cycloalkenes, whereas a hydroboration/ protonation sequence afford ( )-cycloalkenes (Equation (23)). Recently, Trost reported an alternative procedure for the synthesis of (E)-olefins from alkynes through hydrosilylation by a ruthenium catalyst. This procedure converts cycloalkyne 130, for example, to vinylsilane 131 and then to (E)-cycloalkene 132 in a stereoselective manner (Scheme 46)7 ... 

A series of 3-hydroxybut-4-ynoic esters (319) has been prepared from alkynic ketones and bromoacid esters in the presence of zinc (61AP234). Ring closure to the pyran-2-one occurs in a mixture of acetic and sulfuric acids. The preparation of 4,6- and 3,4,6-substituted pyranones is possible by this route, the substituents being different or identical, and as such is complementary to the alkynic route to the 3,6-disubstituted compound (57JA2602). 

Similarly, 1,1-dioxides are formed either by the cyclodehydration of diacylsulfones and ammonia (Scheme 115) (720S(52)135) or through the action of sodamide on bis(2-chlorovinyl) sulfones (255) which are themselves obtained by a two-step sequence from alkynes (Scheme 116) (72S311). A less general route involves the addition of the thiirene sulfone (256) to the mesoionic oxazolone (257). In this example the initial adduct (258) eliminates carbon dioxide spontaneously, thus affording the 1,4-thiazine (259) (75CL1153). 

The alkyne is formally derived by a 1,4-dehydrochlorination with ring opening, although subsequent work has shown that the cyclopropene ring-opens even at ambient temperature to 3-chloro-3-methylbut-l-yne and so a 1,2-dehydrochlorination of this is an alternative. The carbene could be formed by several routes from the cyclopropene, and an early experiment with a 14C-labelled compound indicated that the label at C-l of the cyclopropane became C-l of the carbene ... 

The research reported by Hayashi13 14 indicated a highly efficient route for the preparation of ketones from alkynes via hydration ... 

Reaction of (AW-enamino)carbene complexes (= 4-amino-l-metalla-l,3-diene) (CO)sM=C(OEt)-CH = C(R)NHR (Z)-97 (M = Cr, W R, R1 = Ph, Me, i-Pr, n-Bu, CH2Ph, CH2OMe) with an alkyne R2C=CH (R2 = n-Bu, Ph, CH2OMe) affords (stable) l,4-dihydro-4-pyridinylidene complexes 113, which on protonation with HBF4 yield pyridinium compounds 114 (Scheme 45). Pyridines also are obtained by this route from (N//renamino)carbene complexes.91,98... 

Although oxidation of l,l-bis(dialkylboryl) compounds with alkaline hydrogen peroxide proceeds with initid hydrolysis rather than oxidation (equation 16), oxidation with excess MCPBA yields carboxylic acids in good yields. The overall process is an excellent route from 1-alkynes to carboxylic acids (equation 39). ... 

One of the major aspects of these additions is the stereoselection which is achieved to create ( )- or (Z)-vinylmetals from alkynes, or to create erythro or threo structures from alkenes. The latest developments of this approach provide a route to non-racemic, chiral, metallated structures from common organometallics (RLi, RMgX), either by transmetallation with asymmetrically ligated transition metals (as Zr complexes) or simply... 

Substituted vinylsilanes such as ,Z-RCH=CHSiR 3 and R(SiR 3)C=CH2 are a class of organosilicon compounds commonly used in organic synthesis. Many efficient stereo- and regio-selective methodologies for the synthesis of substituted vinylsilanes involving classical stoichiometric routes from organometallic reagents and, more recently, transition metal-catalyzed transformations of alkynes, silylalkynes, alkenes, and simple vinylsilanes have been reported [1-3]. 

Ganem, B. Hydrozirconation-transmetalation a mild, direct route to higher-order vinylic cuprates from monosubstituted alkenes. One-pot synthesis of protected prostaglandins from alkynes and cyclopentenones in situ generation of higher-order cyanocuprates from alkenylzirconium intermediates. Chemtracts Org. Chem. 1991, 4, 44-47. 

Hydrosilylations. Stereodivergent routes to alkenylsilanes from alkynes and hydrosilanes are available. Alkylidenecyclopropanes undergo ring-opening hydrosilylation to afford homoallylic silanes. ... 

Internal vinyl tellurides, which are not accessible via hydroteUuration of alkynes, have been prepared from alkynes through a vinyl borane route. 

Villieras, J., Perriot, P, and Normant, J.F., Simple route from aldehydes to alkynes and 1-chloro-l-alkynes. Synthesis, 458, 1975. 

Metal borylene complexes are another recent addition to the library of metal boron compounds, and these are now finding applications in organic synthesis, initially via their role as stoichiometric borylene transfer agents in the synthesis of borirenes from alkynes [13]. It is expected that additional applications of newly discovered metal borylene complexes will emerge as new routes to them are developed. 

The Pd-catalysed reaction of phenols with propynoic acids offers an atom-economic and green route to coumarins. Although the reaction with propynoic acid itself is not regioselective, 3-phenylpropynoic acid gives a single product from w-substituted phenols <04S1466>. The synthesis of coumarins both from alkynes, CO and iodophenols and by the reaction of phenols with propiolic esters is facilitated by Co/Rh nanoparticles <04SL2541>. 

The elimination route to alkynes involves removal of two moles of hydrogen halide by strong base from a dihaloalkane. Thc most common kind of. sequence involves forming the dihaloalkane by addition of halogen to an alkene doitble bond. Because you already know how to make alkcnes. yoit now have access to alkynes in the following way ... 

Photogenerated radical cations derived from alkynes by irradiation in methanol with 1, 4-dicyanonaphthalene have been shown to provide another route to alkyne-... 

One final route to alkyls is the attack of a nucleophile on a metal alkene complex, but we will postpone a detailed discussion to Chapter 5. This route is more useful for the synthesis of metal vinyls from alkyne complexes vinyls are also formed from alkyne insertion into M—H bonds ... 

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