1,3 -dienyl-1 -boronic esters

P-Substituted vinylboronic esters were isolated in fair to high yields from the coupling under classical Heck conditions of vinylBhg with (hetero)aryl iodides and bromides (Table 3.1, entries 1-3). A dienyl boronic ester was obtained by coupling of vinylBhg with a vinyl iodide derived from phenylgly-cine (entry 4). The preparation of a styryl boronic ester with a sulfonamide moiety is described in a patent (entry 5) this product was later used in a Suzuki-Miyaura reaction. ... 

Cycloadditions and Other Additions to Alkenyl-, Alkynyl- and Dienyl Boronic Esters... 

Due to their great versatUity, boron-substituted dienophiles and dienes have emerged as attractive building blocks in Diels-Alder reactions [47]. In this section, we review the most important results related to the [4-i-2] cycloadditions involving alkenyl, alkynyl and dienyl boronic esters and some of their derivatives. 

A boron-tethered (C-B-O) intramolecular Diels-Alder (IMDA) approach has been used to prepare cyclic alkenyl boronic esters 140 (Scheme 19). Thus, reaction of 2equiv of the dienyl alcohol 138 with 137 in THF, in the presence of molecular sieves, provides the corresponding IMDA precursors 139. The IMDS reaction was then accomplished at 190 °C in a toluene solution, with 5 mol% of 2,6-di-fer7-butyl-4-methylphenol as a free radical inhibitor. Transformation of the carbon-boron bond in 140, using standard organoborane reactions, can then afford a variety of functionalized cyclohexene derivatives <1999JA450>. 

Different studies were carried out to develop diastereocontrolled versions of these Diels-Alder reactions. Better results were obtained when an optically pure diol was used as chiral director. The tandem reaction involving a 1,3-dienylboronate derived from tartrate esters proceeded with 70% enantiomeric excess (Scheme 9.35) [76]. The hetero-Diels-Alder reaction of enantiomerically pure (-t)-pinanediol 1,3-dienyl-boronate with an azo dienophile gave the J -configured endo cycloadduct as the single stereoisomer [77]. 

Alkenylboronic acids and esters are also very useful substrates (Equation 71, Figure 1.38) [405], in particular to access substituted olefins and dienyl moieties commonly encountered in several classes of bioactive natural products [282, 406]. To this end, Kishi and co-workers examined the influence of the base, and developed an optimal variant using thallium hydroxide [281]. Recently, allylic alcohols were found to couple directly with alkyl and alkenyl boronic acids without the aid of a base [407]. In rare cases, the Suzuki reaction has been applied to the use of alkylboronic acids [296, 408], including cyclopropylboronic acids [409]. Hitherto notorious for their tendency to undergo [3-hydride elimination, alkyl bromides are now suitable as electrophiles under carefully optimized conditions that even allow Csp -Csp couplings with alkylboronic acids (Equation 72) [410]. The Suzuki reaction has also been applied very successfully in solid-phase chemistry and combinatorial library synthesis [411]. It has been applied industrially [412], especially in medicinal chemistry, e.g. in the production of the antihypertensive drug losartan [195]. 

Few works reported the reactivity of simple l,3-dienyl-2-boronates in Diels-Alder reactions. Unlike their 1-substituted analogues, they showed a high propensity to dimerize even at room temperature, like their ester or sulfone congeners (Scheme 9.38) [83]. A theoretical study of this dimerization showed that this process occurred in a highly regioselective way, giving a new allylboronate that further reacted in a one-pot sequence with aldehydes [84]. 

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