Grignard synthesis, doubl

Dehydrobrommatiom. In a procedure for the preparation of tri-/-butylcyclo-propcnyl fluoroborate (4), Ciabattoni et at. prepared dineopenlylketone (I) by the Grignard synthesis formulated, converted it to the a, -dibromide (2), and effected double dehydrobromination by treatment of this intermediate with potassium t-buloxide. The resulting cyclopropcnonc (3) was then brought into reaction with a solution of commercial t-butyllithium in pentane. The mixture was quenched with water, and the pentane layer was washed, dried, and evaporated on a rotary evaporator. The resulting pale-yellow oil is taken up in ether and treated at 0° under rapid magnetic... 

Carbanion additions occur frequently at carbonyl or nitrile groups and occasionally at carbon-carbon double bonds. The Grignard synthesis of alcohols is an example ... 

At —60° in ether, high yields of phosphonites were obtained provided the reaction mixture was distilled directly and not hydrolyzed (102). Dilute aqueous acid readily hydrolyzes phosphonites—a fact which could reduce the efficiency of the Grignard synthesis (53). Experiments by others have supported these suspicions (100,104,176). Probably of equal importance is the use of tetrahydrofuran as a solvent which appears to improve the yields in the cases cited. Improved solvation of the Grignard reagent and the organophosphorus products may be influential. Double displacement of OR occurs at 40° with dibutyl vinylphosphonite which demonstrates the importance of the temperature parameter (99). Even the reactive phosphinite (17) survives at... 

A further synthesis of 1-alkyl-diaziridines is the addition of Grignard compounds to the NN double bond of diazirines [Eq. (35) ]. The... 

Grignard reactions have been used in the preparation of l-(dialkyl-phosphinyl)- ,3-dienes, e.g. (11), which are intermediates in the synthesis of antiflame polymers. Double-resonance n.m.r. techniques have been applied to the study of the reactions between thiophosphoryl halides and Grignard reagents. For example, the reaction between methylmag-nesium iodide and t-butyldibromophosphine sulphide gave the mixed... 

Alkenyl sulfoxides (42 and 43) were first prepared in optically active form by Mulvaney and Ottaviani , described in an article overlooked by most workers in the field, and a year later by Stirling and coworkers through the reaction of the appropriate vinyl Grignard reagent with sulfinate ester 19. Both groups studied the addition of nucleophiles to the carbon-carbon double bond . More recently, Posner and coworkers reported a similar synthesis of ( )-l-alkenyl sulfoxides, e.g. 44 and In the synthesis of 45,... 

The Pauson-Khand reaction is the Co-induced formation of cyclopentenones from ene-ynes and CO. One impressive example of a domino Pauson-Khand process is the synthesis of fenestrane 6/4-15, as reported by Keese and colleagues [278]. The transformation is initiated by a double Grignard reaction of 4-pentynoic acid 6/4-12, followed by protection of the formed tertiary hydroxyl group to give 6/4-13. The Co-induced polycyclization of 6/4-13 led directly to the fenestrane 6/4-15... 

Double addition of Grignard reagents to A-glycosyl nitrones (336), in a domino fashion, affords hydroxylamines. Their usefulness has been shown with the synthesis of pyrroloazepine (418) via a ring closing metathesis key step (Scheme 2.187) (564). 

Remember, however that CHAOS does not intend to be a database of "synthetic methods" presently available to organic chemists. CHAOS is aimed at finding the intermediate precursors of the "synthesis tree" by means of selected basic disconnections (either "consonant" or "dissonant"). In the author s view, it is of minor importance, for instance, how a "carbene" -resulting from a -(2 + 1) cycloelimination- is generated, or whether a double bond is the direct result of a Wittig reaction or the dehydration of an alcohol formed in a Grignard reaction. In... 

Uncatalyzed addition reactions of Grignard reagents with nonconjugated alkenes and aUcynes are of limited use in synthesis. However, carbon-carbon double bonds substituted by a leaving group, such as an acetate, are susceptible to be displaced by organomagnesium compounds presumably by an addition-elimination pathway. A few examples have been reported (equation 64). 

To achieve synthesis of (-)-arisugacin A [1], (7 J-89 was obtained readily from 2-methyl-l,3-cyclohexanedione 88 in 4 steps with an overall yield of 46%, featuring vinylogous ester formation, Stork-Danheiser double alpha methylation,52,68,69 vinyl Grignard addition followed by acidic work-up,68 and an asymmetric CBS reduction [Scheme 21].70,71... 

When dienones such as 55 are subjected to the epoxidation conditions the electron-poorer C=C double bond is selectively epoxidized. The other C=C bond can be functionalized further, for example, it can be dihydroxylated, as shown in the synthesis of the lactone 56 (Scheme 10.11) [82]. Stannyl epoxides such as 57 (Scheme 10.11, see also Table 10.8, R1 = n-Bu3Sn) can be coupled with several electrophiles [72], reduction of chalcone epoxide 58 and ring opening with alkyl aluminum compounds provides access to, e.g., the diol 59 and to phenylpropionic acids (for example 60). Tertiary epoxy alcohols such as 61 can be obtained with excellent diastereoselectivity by addition of Grignard reagents to epoxy ketones [88, 89]. 

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