The syn and anti forms of acetaldoxime are shown below. Treatment of (70) with either sodium methoxide or potassium t-butoxide gives (71), via the enolate of the α,β-unsaturated ester.90 Similarly, (72) cyclizes to (73) with potassium t-butoxide.91 Cyclization of (74) occurs, however, from the saturated rather than unsaturated enolate to give (75).92 This may reflect the stabilizing effect of sulfur on the enolate or the greater stability of the enolate of (75) relative to that of the alternative product (Scheme 30). The position of a substituent in the main ring relative to the shortest bridge is decisive for the assignment of exo or endo (according to IUPAC: the bridge with the highest locant digits[13] in the bridged ring system). The prefixes sec and tert are used to indicate the substituent environment in a molecule. ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. URL: https://www.sciencedirect.com/science/article/pii/B978008097742300207X, URL: https://www.sciencedirect.com/science/article/pii/B9780128018897000224, URL: https://www.sciencedirect.com/science/article/pii/B9780080523491000500, URL: https://www.sciencedirect.com/science/article/pii/B9780080977423002093, URL: https://www.sciencedirect.com/science/article/pii/B9781890661403500090, URL: https://www.sciencedirect.com/science/article/pii/B0080447058002673, URL: https://www.sciencedirect.com/science/article/pii/B9780128007204000131, URL: https://www.sciencedirect.com/science/article/pii/B008045047400114X, Comprehensive Organic Synthesis II (Second Edition), Condensation Reactions of Carbonyl Compounds, Synthesis: Carbon with One Heteroatom Attached by a Single Bond, Comprehensive Organic Functional Group Transformations, Nucleophilic Species That Form Carbon-Carbon Bonds, Applications I: Main Group Compounds in Organic Synthesis, Comprehensive Organometallic Chemistry III. Have questions or comments? The effect is often the result of cation chelation by the carbonyl oxygen atom and one or more additional basic portions of the reactants. [15] If both rings possess an equal number of segments the ring with the most significant substituent according to the CIP rules is chosen. Typically "spiro" is set as normal.[30]. i.) Preparation of Alkenes: A Practical Approach. 2nd ed. An Na radical donates an electron to one of the P bonds in a carbon-carbon triple bond. In the Molander and Haas242 synthesis of davanone, ketone 214 was treated with LDA and LiCl, then with iodomethane. House has worked out a protocol wherein these enolates are allowed to react with aldehydes to give the corresponding aldols.25 Higher yields of aldol products are obtained when the lithium enolate is generated in ether or 1,2-dimethoxyethane (DME) by reaction of an enol acetate with methyllithium. Examples of such are meso compounds such the tropane alkaloids; the parent compound is tropine, whose systematic name is (1R, 3r, 5S)-8-methyl-8-azabicyclo[3.2.1]octane-3-ol. Such isomers are classified by the syn/anti notation. Enolates are the conjugate bases or anions of enols (like alkoxides are the anions of alcohols) and can be prepared using a base. 11.3.4 Catalytic Hydrogenation of Alkynes, 11.3.3 Catalytic Hydrogenation of Alkenes, Hydrogenation of an Alkyne to a Cis-Alkene, Hydrogenation of an Alkyne to a Trans-Alkene, Ternay, Andrew L. Contemporary Organic Chemistry. For aldol products many types of nomenclature have been proposed. About 20% of unreacted 214 was recovered. 4. corresponding to the number of spiro atoms. syn and anti are always written small and italic, locants (if used) are placed in front of the word and separated by hyphens. The regioselectivity for 216 is attributed to formation of the enolate anion and alkylation away from the bridgehead methyl group. In mer configuration the three ligands span a plane in which the central atom is located. cyclo is a frequently used prefix for all cyclic and heterocyclic compounds. The abbreviation o- (short for ortho, from Greek orthós for upright, straight),[8] m- (meta, Greek (roughly) for between)[9] and p- (para, from Greek pará for adjoining, to the side)[10] describe the three possible positional isomers of two substituents on a benzene ring. Where the counterions are rather obvious but not certainly known, they are included in the formulas but placed in brackets [ ]. Typographically, (R) and (S) are placed in uppercase and italic; the frequently preceding locants, the enclosing round brackets and the commas, on the other hand, as normal. Throughout this chapter the syn/anti nomenclature of Masamune 13 is adopted and defined as shown in (3) and (4). o-, m- and p- (written out ortho-, meta- and para-) are written in lowercase letters and italic. [36] For this purpose, all substituents at the stereocentre are prioritized according to the CIP rules and the substituent with the lowest priority ("D") is pointed backwards (away from the viewing direction). In a few transition metal enolates the cation is associated with a delocalized enolate anion (η3-enolate complexes). Wilcox, in Comprehensive Organic Synthesis II (Second Edition), 2014, Enolates, or oxyallyl anions, are versatile reagents for the formation of α-substituted carbonyl compounds and are therefore important intermediates for the synthesis of complex molecules. If the aldol condensation reaction that produces 344 could be controlled, one diastereomer might be formed in preference to the other, making the reaction diastereoselective. Charles K.-F. Chiu, in Comprehensive Organic Functional Group Transformations, 1995. Alkynes can be fully hydrogenated into alkanes with the help of a platinum catalyst. The descriptors, syn and anti are used to distinguish them. Enolate alkylation with simple aldehydes and ketones does not generally lend itself to enantioselective control due to the planar nature of the enolate π system.239 Inspection of 345 shows that the si-re face (face a) has no more steric hindrance than the re-si face (face b). A. Hosomi, K. Miura, in Comprehensive Organometallic Chemistry III, 2007, Silyl enolates react with acyl cation equivalents to give the C- and/or O-acylated products (Equation (90)).333 Fluoride-catalyzed reaction using acyl fluorides is valuable for O-acylation of silyl enolates derived from aldehydes and ketones.334 CuCl also promotes the O-acylation with acyl chlorides.335 The CuCl-promoted reaction of ester silyl enolates results in exclusive C-acylation. At the carbonyl C-atom, the OM (oxy-metal) group is defined to be of highest priority without regard to the nature of the metal,1 for example, (1) and (2). If the residue located on this carbon atom (usually an OH group) points to the left, the molecule originates from the L-series. We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. In general, enolate alkylation proceeds by approach of the enolate to the halide from the less sterically hindered face of the enolate anion. For more information contact us at info@libretexts.org or check out our status page at https://status.libretexts.org. Enolates, formed by the abstraction of the α hydrogen atom by a strong base, are nucleophiles. The typographic presentation of cis and trans is italicised and in lower case letters. Thus, not the exact position of the substituent is described but only the substitution pattern of the adjacent atom (usually a carbon atom). The chiral C-atom most remote from the group with the highest priority is used for the assignment of D- or L-. The enol ester or silyl enol ether route to enolates has advantages over direct deprotonation in certain cases. Williams, Jonathon M.J. [28][29], The prefix "spiro" followed by a Von-Baeyer descriptor describes in the nomenclature of organic compounds ring systems linked by only one common atom, the spiro atom. syn and anti are always written small and italic, locants (if used) are placed in front of the word and separated by hyphens. In general, enolate alkylation reactions produce chiral, racemic products. A descriptor is in chemical nomenclature a prefix placed before the systematic substance name, which describes the configuration or the stereochemistry of the molecule. While n, iso and neo are written in small and italic letters, for cyclo this is only the case in inorganic compounds. This chapter is divided into several sections. The stereochemical descriptors (R) (from Latin rectus = right) and (S) (from lat. In organic structural chemistry, the configuration of a double bond can be described with cis and trans, in case it has a simple substitution pattern with only two residues. When this enolate anion reacts with iodomethane, therefore, no facial selectivity is anticipated and the methylated product will be racemic at the newly formed stereogenic center. Copyright © 2020 Elsevier B.V. or its licensors or contributors. Depending on which catalyst is used, the catalysts cause anti- or syn-addition of hydrogens. For the aldol reaction, ether or mixtures of ether and DME are superior to THF. Because alkynes differ from alkenes on account of their two procurable π bonds, alkynes are more susceptible to additions. Lithium diisopropylamide (LDA) or sodium hydride are required as bases.