The acid-catalysed rearrangement of 1,2-diols

The conversion of pinacol (Expt 5.35) to t-butyl methyl ketone (pinacolone, Expt 5.98) under acid conditions exemplifies a general reaction of 1,2-diols (the pinacol-pinacolone rearrangement). The mechanism, formulated below, involves loss of water from the protonated 1,2-diol accompanied by a 1,2-nucleophilic shift of a methyl group. 

Experiment 5.98 t-BUTYL METHYL KETONE (Pinacolone) Me2C(OH)-C(OH)Me2 Me-CO-CMe3 

In a 500-ml round-bottomed flask carrying a dropping funnel and a connection to a condenser set for distillation, place 50 g of pinacol hydrate (Expt 5.35) and 130 ml of 3 m sulphuric acid. Distil the mixture until the upper layer of the distillate no longer increases in volume (15-20 minutes). Separate the pinacolone layer from the water and return the latter to the reaction flask. Then add 12 ml of concentrated sulphuric acid to the water, followed by a second 50 g portion of pinacol hydrate. Repeat the distillation. Repeat the process twice more until 200 g of pinacol hydrate have been used. 

Dry the combined pinacolone fractions over magnesium sulphate and distil. Collect the pinacolone at 103-107 °C. The yield is 62 g (70%). 

---

Another example of migration of a group, in the original case Me, to a cationic carbon atom occurs in the acid-catalysed rearrangement of 1,2-diols, e.g. pinacol (cf. p. 218) Me2C(OH)C(OH)Me2 (34) to ketones, e.g. pinacolone, MeCOCMe3 (35) ... 

The MNDO method has been employed405 to study the reaction pathway and to optimize the structures of reactant, product, and transition state of the acid-catalysed rearrangement of 1,2-propylene glycol, and the unimolecular dehydration of protonated a,co-diols in the gas phase has been examined406 by tandem mass spectrometric experiments. It has been shown that the reaction of l,2-diarylcyclopropane-l,2-diols (342) with acids yields primarily the a,//-unsaturated ketones (343) in which the aryl... 

The MNDO method has been employed " to study the acid-catalysed rearrangement of propylene 1,2-glycol. Propanaldehyde was found to be the major product with a small amount of acetone also being produced. The solid-state pinacol rearrangement of l,l,2-triphenylethane-l,2-diol has been performed over various solid... 

Complete details of the acid-catalysed rearrangement of humulene 1,2-epox-ide (249) have now been presented. With 1.8M-sulphuric acid in acetone at 0 °C for 30 min the sole product is the previously "known tricyclic diol (250). After an extended period this diol gives rise to five other identified products. 

The acid-catalysed rearrangement of two enantiomerically pure epoxides derived from sesquiterpenic himachalenes with Lewis and Brpnsted acids results in polycyclic compounds and mechanisms to explain the products have been proposed. Treatment of acyclic a-hydroxy and a-tosylamino sulflnyl dienes with amines gives enantiopure 1,4-diol or 1,4-hydroxysulfonamide derivatives.This one-pot proce- (g) dure entails a conjugate addition that triggers a diastereoselective sulfoxide-sulfenate [2,3]-sigmatropic rearrangement (Scheme 61). 

Reactions of aldehydes and their derivatives have been the subject of several reports. Benzene-1,2-dicarboxaldehyde condenses" with 2,3-dihydro-naphthazarone to give the p-quinone (75), and a similar reaction occurs between diketones such as PhCOCH2CH2COPh and naphthalene-1,4-diol. 4-Nitrobenzaldehyde undergoes disproportionation in the presence of cyanide ions and methanol, resulting in methyl 4-nitrobenzoate as the main product. The acid-catalysed rearrangement of arylhydrazones (76) derived from aromatic aldehydes and diaryl ketones leads to amino-biphenyls (77). 

A comparison has been made" of various monoterpenoid rearrangements cataiysed by either zirconium phosphates or by zirconium organo-substituted phosphonates, and acid-catalysed rearrangements of a-trans- and /3-cis-3,4-epoxycaranes have been described." It has been observed" that on exposure to Li (OBu )3AlH, perhydro-naphthalene-l,4-diol monosulfonate ester (269) rearranges to the 11-oxatricyclo-[5.3.1.0.2,6]undecane derivative (270) (see Scheme 88). 

A chiral Lewis acid-catalysed method for the 1,2-migration of (dichloromethyl)borate complexes to provide synthetically useful (a-chloroalkyl)boronates has been developed,557 and the diastereoselective rearrangement of the a,a-dichloromethylboronate derivatives of 1,2-diols, (427) —> (429), has been explained558 on the basis of a bidentate interaction between the catalytic Lewis acid and the substrate, leading to a favoured transition state (428). 

A stereospecific synthesis of 1,2-diols is reported advantage is taken of the clean inversion occurring on acid-catalysed opening of an epoxide with DMSO. f -Sulphoperbenzoic acid (198), a water-soluble per-acid, converts olefins into trans-1,2-diols by spontaneous acid-catalysed cleavage of the intermediate epoxide no esters are observed. The epoxide can be isolated, if desired, by employment of a buffer. This per-acid also effects Baeyer-Villiger rearrangement and oxidizes amines to iV-oxides. 

Some insect pheromones are internal ketals. We have already mentioned multistriatin (pp T 2 and 99) and frontalin p 193). Brevicomin (22) is another example. Disconnection of the ketal gives (23) containing a 1,2-diol. Among other syntheses, hydroxy-lation of protected enone (24) by epoxidation and acid catalysed rearrangement gives brevicomin stereo-specifically,... 

Naya and Kotake, in an examination of Japanese hop oil, have isolated three humulane-type compounds, viz., humuladienone (161, R = Me), humulenone II (161,R = =CH2), and humulol (162), in addition to the tricyclic diol (163, R = OH), m.p. 207 °C. This diol has already been prepared in two different ways (a) Sutherland et treated humulene (164) with AT-bromosuccini-mide in aqueous acetone and converted the resultant bromohydrin (163, R = Br) to the diol (163, R = OH), m.p. 205—206 °C, by hydrolysis, (b) McKervey and Wright obtained the same diol, m.p. 201—203 °C, by acid-catalysed (20% sulphuric acid) rearrangement of humulene 1,2-epoxide (165), a known natural product. On the basis of these findings and the fact that both caryophyllene (166) and humulene can be derived from the above bromohydrin by two in vitro steps, McKervey and Wright postulated that humulene 1,2-epoxide may be involved in the biosynthesis of the tricyclic diol and caryophyllene. This postulate does not, however, readily accommodate the observed rotations of the relevant... 

The transformation of enantiomerically enriched 1,1,4-trisubstituted but-2-yne-l,4-diols (262) into 2,2,5-trisubstituted 3-acyloxy-2,5-dihydrofurans (267) with complete stereospecificity has been achieved by an Ag(I)-mediated rearrangement of the monoesters (263) to allenic intermediates (265), followed by Ag(I)-assisted cycUzation. A possible mechanism for the rearrangement and cyclization is shown in Scheme 66, in which the carbon-oxygen bond (b) in (266) is formed from the back side of the carbon-oxygen bond (a) in (264). A novel synthetic method has been developed for the rearrangement of 3-aryl-l,2-dialkynylallyl alcohols into m-enediynes under mild acidic conditions. The allylic rearrangement has been shown to involve an acid-catalysed isomerization step to convert the allyl alcohol into an allylic cation... 

---