RewriteEngine On
RewriteBase /

RewriteCond %{THE_REQUEST} ^[A-Z]{3,}\s/+folder/([^\s]+) [NC]
RewriteRule ^ %1 [R=301,L]

RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule (?!^folder/)^(.*)$ /folder/$1 [L,NC]