Boronic chain extension with

The Ci9 conjugated aldehyde component was reacted with the lithium derivative of this Cg acetal in ammonia to afford an hydroxy compound which was dehydrated under mild acidic conditions to the fully conjugated C25 substance. Chain extension with ethyl vinyl ether in the presence of boron trifluoride-zinc chloride and mild acidic deethanolation gave the Cjt acetal which was then converted with prop-1-enyl ethyl ether under the same conditions to the required C30 structure of dehydro-p-apocarotenal. Lindlar partial reduction followed by isomerisation afforded the final product. The route is shown in Scheme 14b Scheme 14b... 

If more substituents are desired, the precursor to the cyclobutane can be assembled with more chain extensions with (dichloromethyl)lithium. For example, the (bromomethyl)boronic ester 50 is easily prepared in lots >300 g 26). Alkylation with lithiopropionitrile yielded 51, which was transesterified to astereomeric mixture 52, converted via 53 to in the usual manner, and again treated with (dichloromethyl)lithium to form 55. An analogous series without the methyl substituent was also prepared from lithioacetonitrile. Treatment with LDA led via 56 to cyclobutaiKS 57. This woric was done before the role of magnesium salts in the ring closure was understood, and yields of 57 were consequently not optimized (Scheme 13). 

The stability of a-azido substituents in boronic esters was first observed in exploratory studies [29]. Azido groups are compatible with several standard reactions of boronic esters, including chain extension with (dichloromethyl)lithium and substitution of the resulting a-chloro substituent. After peroxidic deboronation, reduction of the azido group with lithium aluminum hydride led to an asymmetric amino alcohol, (5S,6S)-BuCH(NH2)CH(OH)Bu, in 98% diastereopurity [29]. Details ofa more recent amino alcohol synthesis are shown below in Scheme 8.31. 

A useful application of this chemistry has been reported for the preparation of an intermediate amino alcohol (134) used in the total synthesis of the phosphatase inhibitor motuporin (Scheme 8.31) [69]. Straightforward chain extensions of (R)-pinanediol (p-methoxybenzyloxymethyl)boronate (131) to (a-bromoalkyl)boronic ester 132 followed by azide substitution furnished (a-azidoalkyl)boronic ester 133, which was converted into 134 by chain extension with LiCHjCl followed by the usual peroxidic deboronation and azide reduction. 

For the synthesis of amino acids, the reaction of an a-haloalkyl boronic ester 4 with sodium azide and a phase-transfer catalyst in dichloromethane/water requires a large excess of azide in order to form the a-azidoalkyl boronic ester 5 with only 1-2% epimer34. With the exception of R1 = benzyl, where epimerization of 4 is relatively rapid, bromoalkyl boronic esters are preferred. Chloroalkyl boronic esters react so slowly that the azide and dichloromethane may generate hazardously explosive diazidomethane65,66. Chain extension of 5 to 6 proceeds normally. Sodium chlorite, which is known to oxidize aldehydes to carboxylic acids67-69, also oxidizes a-chloroalkyl boronic esters to carboxylic acids34. The azido acid is hydrogenated to the amino acid. 

Application of the previously described reaction sequence allows an initial insertion reaction with dichloromethyllithium to give 20 Then 20 is enhanced by one C, residue using Grignard reagent 21 to produce 22. Another reaction with dichloromethyllithium results in a C chain extension to product 5. Since the insertion reaction with dichloromethyllithium was conducted twice, the distance between the boron atom and the PMB ether inemases by two carbon atoms. 

Chain extensions using an insertion reaction of dichloromethyllithium or dibromomethyllithium with (5 )-pinanediol [(benzyloxy)methyl]boronate 26 has been used to generate L-C3-, L-C4-and L-Cs-aldoses [51]. In order to obtain 2,3-di-O-benzyl-L-glyceraldehyde 27, the insertion reaction has to be applied twice (Scheme 13.20). By repeating the process two more times, L-ribose has been prepared with high enantiomeric purity [51]. 

Dimethylsulfoxonium methylide. 14,152 15,147 16,146 17,126-127 18,148 19,139 Polyhomologation. The ylide provides the methylene unit in chain extension of triorganoboranes. Besides oxidative workup to generate alkanols, the replacement of the boron atom by a hydroxylated carbon on reaction with dichloromethyl methyl ether gives compounds with trident carbon chains. 

Boron-mediated asymmetric aldol condensation methodology developed by Evans [90] served as an inspiration for preparation of daunosamine starting from chiral oxazoUdinones. It appeared that the choice of chiral auxiUary is quite important for the stereochemical outcome of planned reactions [91]. A successful series of reactions started from N -succinoylation of (R)-3-(l-oxo-3-carbomethoxypropyl)-4-diphenylmethyl)oxazolidin-2-one as a novel chiral auxihary. The chain extension was achieved in aldol condensation with protected lactaldehyde and the key intermediate 132 was converted into the target aminosugar 135, via Curtius rearrangement of carboxyhc acid azide, and reduction of lactone to lactol, as depicted in Scheme 24 [58]. Unexpectedly, boron catalysts were rather ineffective in the aldol condensation step and had to be replaced with more reactive lithiiun enolates (which proved to be non-Evans syn selective). 

There are several situations where cleavage of a 1,3,2-dioxaborolane to the boronic acid and diol is useful. One of these is for removal of a chiral director and replacement by its enantiomer. The first time we encountered this problem, a pinanediol ester was converted to the boronic acid via destructive cleavage of the pinanediol with boron trichloride (14). More recently, it has proved possible to convert an (R)-DICHED a-benzyloxy boronate (20) to the free bororric acid (23) with the aid of sodium hydroxide and a tris(hydroxymethyl)methane to form water soluble derivative 21 (R = CH2OH or NH(CHi)3S03 ) plus water insoluble (R)-DICHED (22). Treatment of 23 with (S)-DICHED (24) then yielded diastereomer 25 (76%, 97-98% diastereomeric purity). Further chain extension and alkylation led to 26 and 27, and deboronation yielded 28, all of which are stereoisomers that could not be accessed directly with a single chiral director (Scheme 6). 

Cyclobutane synthesis allows introduction of substituents on the cyclobutane ring in various patterns (Scheme 8.24) [55], Allyl bromide with boron trichloride and tri-ethylsilane yields the alkyldichloroborane 103, which is converted into pinacol (3-bro-mopropyl)boronate (104) and on to the cyano derivative 105 by standard methods. Transesterification of 105 and reaction with LiCHClj was used to make 100. However, 105 can be deprotonated and monoalkylated efficiently, and transesterification then yields 106. Transesterification with DICHED and asymmetric insertion of the CHCl group furnishes 107, which cyclizes to 108 or 109 with about the same 20 1 di-astereoselection as seen with the unsubstituted intermediate 100. The pattern of substitution shown by 111 was achieved via reaction of pinacol (bromomethyl)boronate (63) with lithioacetonitrile to form 110, which underwent chain extension and substitution in the usual manner. It was necessary to construct 110 in this way because substitution of a (p-haloalkyl)boronic acid is not possible. With R = H or CH3, substituents included Me, Bu, and OBn [55]. 

Chain extension using an insertion reaction of dichloromethyl-lithium or dibromomethyllithium with a cyclic chiral boronate derivative, (S)-pinanediol[(benzyloxy)methylJboronate (2), has been used in a synthesis of L-ribose in 13% overall yield (Scheme 3)- A new... 

The material known as bouncing putty is also a silicone polymer with the occasional Si—O—B group in the chain, in this case with 1 boron atom to about every 3-100 silicon atoms. The material flows on storage, and on slow extension shows viscous flow. However, small pieces dropped onto a hard surface show a high elastic rebound, whilst on sudden striking they may shatter. The material had some use in electrical equipment, as a children s novelty and as a useful teaching aid, but is now difficult to obtain. 

Unsubstituted alkyl azides react with nitrosonium tetrafluoroborate without formation of alkyl fluorides, but azidonitriles give fluoronitriles with this reagent.69 Depending on the chain length, rearrangements can occur extensively. Interaction between the nitrile group and boron trifluoride has been invoked to explain these reactions. 

We have described the structures of some boron-rich borides in which there are extensive 3D systems of B—B bonds. At the other extreme there are crystalline borides with low boron content in which there are isolated B atoms, that is, B atoms surrounded entirely by metal atoms as nearest neighbours. With increasing boron content the B atoms link together to form first B2 units, then chains, layers, or 3D frameworks extending throughout the whole crystal (Table 24.3). 

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