absence of polar groups on the α carbon, and stereoselectivity increases as the size of the achiral ketone substituent (R) increases. Transition state III is favored for reductions of alkyl ketones in which RM is an electron-withdrawing group, because the nucleophile and electron-withdrawing substituent prefer to be as far away from one another as possible. Diastereoselectivity in reductions of cyclic ketones has also been studied. Conformationally flexible ketones undergo axial attack by the hydride reagent, leading to the equatorial alcohol. Rigid cyclic ketones, on the other hand, undergo primarily equatorial attack to afford the axial alcohol. Preferential equatorial attack on rigid ketones has been rationalized by invoking "steric approach control"—an equatorial approach of the hydride reagent is less sterically hindered than an axial approach. The preference for axial attack on conformationally flexible cyclic ketones has been addressed by a model put forth by Felkin and Anh. The transition state for axial attack (IV) suffers from steric strain between any axial substituents and the incoming hydride reagent. The transition state for equatorial attack (V) suffers from torsional strain between the incoming hydride reagent and adjacent equatorial hydrogens. The difference between these two strain energies determines which direction of attack is favored, and when R is small, torsional strain in V dominates and the equatorial alcohol product is favored. Scope and limitations Alkoxyaluminium and closely related hydride reagents reduce a wide variety of functional groups, often with good selectivity. This section, organized by functional group, covers the most common or synthetically useful methods for alkoxyaluminium hydride reduction of organic compounds. Many selective reductions of carbonyl compounds can be effected by taking advantage of the unique reactivity profiles of metal alkoxylaluminium hydrides. For instance, lithium tri-tert-butoxy)aluminium hydride (LTBA) reduces aldehydes and ketones selectively in the presence of esters, with which it reacts extremely slowly. α,β-Unsaturated ketones may be reduced selectively in a 1,2 or 1,4 sense by a judicious choice of reducing agent. Use of relatively unhindered lithium trimethoxyaluminium hydride results in nearly quantitative direct addition to the carbonyl group (Eq. ()). On the other hand, use of the bulky reagent LTBA leads to a high yield of the conjugate addition product (Eq. ()). Ether cleavage is difficult to accomplish with most hydride reagents. However, debenzylation of benzyl aryl ethers may be accomplished with SMEAH. This protocol is a useful alternative to methods requiring acid or hydrogenolysis (e.g., Pd/C and hydrogen gas). Epoxides