et al. # a comprehensive database of paleoclimate records is needed to place recent warming into the longer-term context of natural climate variability. We present a global compilation of quality-controlled, published, temperature-sensitive proxy records extending back 12,000 years through the Holocene. Data were compiled from 679 sites where time series cover at least 4000 years, are resolved at sub-millennial scale (median spacing of 400 years or finer) and have at least one age control point every 3000 years, with cutoff values slackened in datasparse regions. The data derive from lake sediment (51%), marine sediment (31%), peat (11%), glacier ice (3%), and other natural archives. The database contains 1319 records, including 157 from the Southern Hemisphere. the multi-proxy database comprises paleotemperature time series based on ecological assemblages, as well as biophysical and geochemical indicators that reflect mean annual or seasonal temperatures, as encoded in the database. This database can be used to reconstruct the spatiotemporal evolution of Holocene temperature at global to regional scales, and is publicly available in Linked Paleo Data (LiPD) format.

SummaryThree methods are described by which diastereomerically enriched nitroaldols and their O-silylated derivatives can be prepared. threo-Nitroaldols prevail up to 10: 1 over the erythro-isomers if doubly deprotonated nitroaldols 28 are quenched with acetic acid (THF/HMPT or DMPU, -looo) (see Scheme 5 and Table 2). O-Trimethyl-or O-(f-butyl)dimethylsilylated (TBDMSi) erythro-nitroaldols can be obtained by protonation of the corresponding lithium nitronates (35, 39) in THF at low temperature (see Schemes 6 and 7). The erythro-O-TBDMSi-nitroaldol derivatives are also formed in the fluoride catalyzed addition of TBDMSi-nitronates (40-45) to aldehydes (see Schemes 8 and 9). In the latter reaction no 1,2-asymrnetric induction is observed if a-branched silylnitronates or aldehydes are employed (see 48/49 and 50/51). -The stereochemical course of the reactions leading to erythro-O-TBDMSi-nitroaldols follows topological rules of broad applicability (see Scheme 10); possible mechanisms are discussed. -The configuration of erythro/ threo-nitroaldols is determined by chemical correlation (see 24-26) and by I3C-NMR. spectroscopy. -Some examples of the preparation of diastereomerically enriched 1,2-aminoalcohols by reduction of the corresponding nitro compounds without loss of configurational purity are described (see Schemes I 1 and 12).A) Introduction. -The nitroaldol-or Henry reaction is one of the classical C,Cbond forming processes. It furnishes the 1,2-functionalized nitroalcohols 1, precursors of the symmetrical (R' = R2) and unsymmetrical (R' # R2) aminoalcohols 2.Nitroaldols have been extensively reviewed [3] [4], and have frequently been used as intermediates in synthesis. However, the lack of stereoselectivity in the Henry reaction (except in cyclic systems) has hardly been mentioned. This lack of selectivity is due to the reversibility of the reaction and the easy epimerization at the nitro-substituted C-atom. The nitroaldols of type 1 (R',R2#H) occur in two dia-

Precipitation is predicted to increase in the Arctic as temperature increases and sea ice retreats. Yet the mechanisms controlling precipitation in the Arctic are poorly understood and quantified only by the short, sparse instrumental record. We use hydrogen isotope ratios (δ2H) of lipid biomarkers in lake sediments from western Greenland to reconstruct precipitation seasonality and summer temperature during the past 8 kyr. Aquatic biomarker δ2H was 100‰ more negative from 6 to 4 ka than during the early and late Holocene, which we interpret to reflect increased winter snowfall. The middle Holocene also had high summer air temperature, decreased early winter sea ice in Baffin Bay and the Labrador Sea, and a strong, warm West Greenland Current. These results corroborate model predictions of winter snowfall increases caused by sea ice retreat and furthermore suggest that warm currents advecting more heat into the polar seas may enhance Arctic evaporation and snowfall.