The epimeric 9-oxobicyclo[3.3.1]nonane-3-carboxylic acids: hydrogen-bonding patterns of theendoacid and the lactol of theexoacid

Abstract

The two delta-keto carboxylic acids of the title, both C(10)H(14)O(3), are epimeric at the site of carboxyl attachment. The endo (3alpha) epimer, (I), has its keto-acid ring in a boat conformation, with the tilt of the carboxyl group creating conformational chirality. The molecules form hydrogen bonds by centrosymmetric pairing of carboxyl groups across the corners of the chosen cell [O...O = 2.671 (2) A and O-H...O = 179 (2) degrees ]. Two close intermolecular C-H...O contacts exist for the ketone. The exo (3beta) epimer exists in the closed ring-chain tautomeric form as the lactol, 8-hydroxy-9-oxatricyclo[5.3.1.0(3,8)]undecan-10-one, (II). The molecules have conformational chirality, and the hydrogen-bonding scheme involves intermolecular hydroxyl-to-carbonyl chains of molecules screw-related in b [O...O = 2.741 (2) A and O-H...O = 177 (2) degrees ].

Two (+)-α,4-dimethyl-2-oxocyclohexaneacetic acids: hydrogen bonding in a terpenoid γ-keto acid and in a diastereomeric lactol

“…Our study of hydrogen bonding in solid ketocarboxylic acids has included several -and -keto acids that crystallize as lactols (Thompson et al, 1985;Papadakis et al, 2003). We now report results for a diastereomeric pair of -keto acids having the title structure, of which one crystallizes in the open-chain and the other in the lactol form.…”

“…With no intramolecular hydrogen bonding possible, the molecule adopts the intermolecular mode commonly seen in such lactols, a hydroxyl-to-carbonyl catemer. As happens frequently (Papadakis et al, 2003), the units of the chain are screw-related, in this case following the b axis in both directions. We characterize the geometry of hydrogen bonding to carbonyl groups using a combination of the HÁ Á ÁO C angle (ideal value 120 ) and the HÁ Á ÁO CÐC torsion angle (ideal value 0 ).…”

“…Although the factors affecting this tendency have been studied (Soffer et al, 1950;Jones, 1963), the open and closed forms often lie so close energetically that small changes in the structure or the medium can shift the equilibria appreciably (Valters & Flitsch, 1985), so that predictions regarding equilibrium values for speci®c cases remain hazardous. Some such keto acids crystallize exclusively as lactols (Thompson et al, 1985;Papadakis et al, 2003). However, with low energy barriers, the equilibria involved may shift even during crystallization, so that the The asymmetric unit in (II), with skeletal numbering identical to that for (I).…”

The Preferred Ring-Tautomeric Form of a Bicyclic γ-Ketocarboxylic Acid: An Equilibrium Driven by Relief of Angular Hybridization Strain

“…Many of the -and -ketocarboxylic acids we study are capable of ring-chain tautomerism [4,5] and several are known to exist predominantly or exclusively in the closed, lactol form, sometimes referred to as the "pseudoacid" [6][7][8][9][10]. A search of the X-ray literature (Cambridge Structural Database, Version 5.27, update of May, 2006) [11] reveals some 63 examples of keto-acid lactol structures, of widely varying degrees of complexity.…”