Severity: Warning
Message: file_get_contents(https://...@gmail.com&api_key=61f08fa0b96a73de8c900d749fcb997acc09&a=1): Failed to open stream: HTTP request failed! HTTP/1.1 429 Too Many Requests
Filename: helpers/my_audit_helper.php
Line Number: 176
Backtrace:
File: /var/www/html/application/helpers/my_audit_helper.php
Line: 176
Function: file_get_contents
File: /var/www/html/application/helpers/my_audit_helper.php
Line: 250
Function: simplexml_load_file_from_url
File: /var/www/html/application/helpers/my_audit_helper.php
Line: 3122
Function: getPubMedXML
File: /var/www/html/application/controllers/Detail.php
Line: 575
Function: pubMedSearch_Global
File: /var/www/html/application/controllers/Detail.php
Line: 489
Function: pubMedGetRelatedKeyword
File: /var/www/html/index.php
Line: 316
Function: require_once
The highly efficient enantioselective [4 + 2] cycloaddition of o-QMs with ortho-hydroxyphenyl-substituted α,β-unsaturated compounds was realized by using a chiral N, N'-dioxide-Sc(III) complex as catalyst. A variety of chiral chromane derivatives with three continuous stereocenters were obtained in excellent results (up to 99% yield, >19:1 dr, and 99% ee) under as low as 0.005-1 mol % catalyst loading. Besides, a catalytic cycle with a possible transition state model was proposed to elucidate the origin of the chirality.
Download full-text PDF |
Source |
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http://dx.doi.org/10.1021/acs.joc.8b01425 | DOI Listing |
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